On Friday I woke as I came into contact with the ground. Over the night, the air had slowly leaked out of my inflatable mattress until eventually it was possible to feel every bump on the grass below. The rain was falling on the canvas with a pattering sound. At my feet lay the previous night's dinner: a half-eaten plate of salad with a couple of cigarette ends in it. My mobile phone beeped. Svein Gustaf, who was working on the mainland that morning, wanted to know what was going on.

"Where are you? How was Gro?" he asked.

"No, no, no," I mumbled.

It was past 1pm.

Gro Harlem Brundtland's speech was one of the main highlights of the camp. Now it had finished. I unzipped the tent door and stuck out my head. Camp members passed along the path outside.

On the way back from the showers, I almost walked into a small retinue of reporters. Striding among them was a figure in a scarlet raincoat and green wellies. It was the mother of the nation herself, Gro, Norway's first female prime minister. So she was still here. And she still had to check out of the information office before she left.

It was complete chaos there. The queue to the desk stretched back to the door. Binders, papers and paper cups lay on tables and chairs. In the back room phones rang continuously. I barely had time to ask if there was something I could do before Gro came bursting through the door with her entourage and disappeared upstairs for lunch.

We always had two walkie-talkies in the office. I took one in case anything came up. Then we received a message from the other shore about a badly parked car. You normally had to hand in your car keys before you got on the MS Thorbjørn [the boat between the island and the mainland], so it could be moved if necessary, but this person hadn't done that. We had to find out who it was. I went to the back room and called for the owner over the public address system. When I came back out, Mari was there. In the commotion around Gro, she had somehow managed to sprain her ankle; it was twice the size of the other one. She agreed to sit for a little until the swelling had gone down, while the rest of us postponed football and volleyball matches due to the rain.

At 3pm, Gro left and we sat around chatting, sharing leftover sandwiches. The last few days, a young, blond-haired boy had been popping by to ask if anyone had found his jacket. It was half past three when he came through the door that day, just as Monica came barging in from the other side.

"There's been an explosion in Oslo," she said.

Everyone stopped talking and turned towards her. It had to have been a gas explosion, I told myself. Nobody would set off a bomb in Oslo, in little Norway. But thoughts buzzed around at the back of my head about Norway's participation in the war in Afghanistan, about all the fighter aircraft we had sent to Libya. We had taken risks.

Monica ran up to the first floor to find a radio. She tried to tune in to a news channel, but it was five minutes before we received the first report. At the same time, the Aftenposten [newspaper] website described shattered glass, alarms blaring and smoke coming from the main government building. Some people began to cry.

The Labour party Workers' Youth League (AUF) leaders decided to hold a general meeting. It was announced that everyone was to meet in the main hall at 4.30pm. At about the same time, I received a message from Svein. "Don't go to the centre of Oslo," he wrote. "There might be more bombs."

"It could be gas, but I'm not sure," I replied.

When Svein answered, I had already taken the walkie-talkie and left the office to gather people for the meeting. At this point there was nothing to suggest that anyone other than people in the centre of Oslo had anything to watch out for. All the same, he wrote, "Be careful, Adrian."

It was 4 o'clock.

The smell in the corridor outside the main hall was intense. The weather had been wet and the odour of several hundred damp boots and shoes filled the narrow passageway. Inside, the windows were misted up and the air was dank and muggy. Eskil Pedersen, the AUF leader, had already begun his briefing. "As you have probably heard, there has been an explosion in the government quarter," he said. "We're OK here. This is the safest place we can be right now."

Monica took the floor. The disco that evening was to be put on hold, but otherwise the camp would continue as normal. We were on an island far from Oslo. It felt like the most sheltered place in the country at that moment. Monica explained that the MS Thorbjørn would cancel all departures, sailing instead as required so that those who wished to could leave the island at any time.

It was a quarter to five.

Before we separated, the delegations gathered for their own meetings. My branch met at the outdoor stage. "It might be a good idea to call your parents," I said, "to tell them you're all right." On the way back to the office, I checked my phone: there were reports of two deaths in Oslo. On the walkie-talkie, they notified us from the shore that a policeman was waiting for the boat to come.

It was 5 o'clock.

In the office, the mood had turned from shock into grief and despair. Mari was one of those who took it hardest. Cheerful and energetic Mari. When she had composed herself, she asked if I could go and buy some fizzy drinks and crisps in case anyone came and wanted to talk.

"Of course," I said.

She wanted to give me her card, but couldn't find it. "It's fine," I said. She smiled and thanked me.

The first thing many people did was call home. I hadn't made that call yet. Mum was in Poland, and is quick to become anxious. It felt unnecessary to trouble her before we knew any more, but it turned out she had seen the news on TV. When she rang, I was on my way out of the main house.

"Where are you?"

"I'm on Utøya."

She still seemed nervous. "Are you all right?"

On the lawn outside, they had installed two big bouncy castles. I snuck in between them and lit a cigarette. "Relax," I said. "I'm fine."

"What are you going to do now, Adrian?"

From where I was standing, you could see the fjord between Utøya and the shore: 550m of water separating the place from the rest of the world. Before the incident in Oslo, it had been part of the place's magic. Part of what made a summer camp there a special experience, as if we were a small, autonomous society. Now it felt more as if we were cut off, and the distance made it even more difficult to understand what was happening.

"I'm not going home," I said. "I'm on an island anyway."

"You're probably right," said Mum.

It was a quarter past five.

An aerial view of Utøya Island, Norway, taken on July 21 2011, shortly before police sent anti-terror police to investigate reports that a man dressed in a police uniform had opened fire at a youth camp there. Photograph: AP

Next to the storehouse, I met a boy from the emergency team. "Have you heard the police are coming?" he asked. We stopped and peered out at the water. The MS Thorbjørn was about halfway across the strait. I turned along the steep path up the slope. As I walked, I heard two sharp bangs, as if someone were hammering a sheet of metal. The fact that they came from the direction of the main house made them less remarkable – if someone was working on something, it was usually down there. Soon afterwards, there came another series of bangs. I craned my neck, but from where I was standing, it was impossible to see the office.

It fell silent for a few seconds. Then people came running, some straight up the short cut, others along the track. There was something about the way they were running. Why were they turning their heads? Why did they stop to turn round, then keep rushing onwards? I remained still until I saw Tonje. She came dashing up the short cut. As she came closer, she fixed her gaze on me.

"Run!" she shouted.

"Why?" I asked.

"He's shooting!"

I began to hurry along behind her, but slowed down when I came to the bottom of the short, steep hill between the cafe and the campsite. Tonje disappeared into the woods. Around me, several others stopped and for a moment we stood there looking at each other in confusion. Our curiosity fought against our fear. Suddenly, someone opened a window in the main hall. Kahlid stuck out his head: "What's going on?"

"Close the windows and lock the doors," I said.

Straight away there came another bang. This time it was much louder. Five metres farther up, I saw a girl run towards me on the track; another two ringing shots and she fell forwards and stayed lying there. Out of the corner of my eye, I saw a boy collapse on top of her. What was this? An exercise? Role-play? I was moving backwards in between the tents, along a path trodden through the muddy grass, when I saw him for the first time.

He came out from behind the trees and on to the track where the girl had fallen. His back was straight, his steps quick and determined, but there was something dispassionate about the way he responded to all the people fleeing around him. He was on his way towards me and I could see he was dressed in dark clothes, with his trousers tucked into his boots, in something that resembled a police uniform. Over his shoulder he was carrying a bag. The track he was on passed barely five metres from where I was standing, and the tent I was crouching behind reached only to my chest. He didn't register me. His gaze was fixed somewhere in front, by the entrance to the small assembly hall. Everything else seemed unimportant or invisible to him.

Maybe he had already spotted the girl in the grey jogging bottoms – I have no idea where she came from, whether she'd been standing there the whole time or had come from the showers. She couldn't have realised what had just happened, because she walked straight towards him. And he walked to meet her. When she was scarcely a metre away, about the distance at which it would have been natural for one of them to say something, he raised a pistol with his right hand and aimed it at her. She stepped back, but it was too late. He shot her three times in the torso. Then he stood over her, looked at her briefly, and let off another two shots. Every time he hit her, her arms and legs twitched.

At the same moment, everyone around me began to run in panic. Out of pure instinct, I set off after them, between the tents, out on to the track that divides the campsite in two and on down towards the woods. We could no longer hear the shots as they left his weapon, but when the bullets hit the trees they split the bark with a brittle, shattering noise. We crouched down as we ran. By the thicket, a girl fell over. At first I thought that, like many others, she'd stumbled on the taut guy ropes that crisscrossed between the tents, but this girl stayed down, and when I turned round I thought I saw a hole in her back.

Up by the cafe, the policeman was on his way among the tents. We had to keep moving between the trees and through the woods. How we got from that point down to the water is still difficult to understand. Not long after, there rose a thick wall of thorns and scrub. We must have run through it, ploughing it down like a herd of cattle. On the other side, the vegetation opened up again and the slope led down towards a small outcrop wreathed with pine trees. We were heading for the water, sometimes in a straight line, at other times zigzagging to avoid what we felt was the killer's gun sight on our backs. As we ran, we shouted to each other: "We've got to call the police!"

We continued down a steep slope that ended on the foreshore. I passed Tim there. He was lying with his face in the gravel.

"You can't hide here!" I shouted.

He peered up in bewilderment and got back on to his feet. A line of blood was running from his forehead. He had probably run straight into a branch or tree. A couple of metres out in the water was a long line of broad-leaved trees, and the only way of going farther without getting your feet wet was through the narrow passage between the trees and the land. When people came rushing down from the slope, most ran through there, but it was also possible to walk around the outside.

I stood there, hesitating.

Well hidden among the rocks behind the trees was a roll of chicken wire. Several people fleeing got their tired feet caught in it and tumbled to the ground. Behind them, the group came to a halt. At any point he could emerge at the top of the hill or come along the path. With unsteady footsteps, I went out on to the slippery rocks to go around the outside of the trees. With every step, the water rose up round my ankles. Then, in a flash, it dawned on me: this is actually happening. This is real. On the way down to the shore I had been expecting somebody to blow a whistle and call a halt to the whole thing. But as the icy water from Tyrifjorden crept inside my boots, it was like a wake-up call going directly from my feet to my brain. A message from the body to the head to keep running. A young boy in a brown hoodie caught up with me. "There's a man shooting," he said.

"Yes, I know," I said. "But why?"

Nobody was thinking about the terrorist attack in Oslo now. I picked up my phone to call the police. There was no dialling tone, just three short pips before it was cut off. They're in on it, I thought. They've already been notified! If only I had got through, they would surely have been able to tell me that I didn't have to swim, that it was only an exercise. That everything would be all right.

We were at the southernmost tip of Utøya. The row of trees and bushes sticking out of the shallow water continued more or less unbroken until it ended at a pointed headland 100m farther down. A stream of friends led all the way from where I was standing to that point. When they came to the headland, they stopped, undressed and, one by one, waded out into the dark water and started swimming. Some had already got a distance away; I could glimpse their heads as they bobbed up and down with every stroke. The temperature in the lake was much lower than normal due to the large amount of rain over the previous weeks. My body ached after the long run, but I thought that was where I had to go. I had to get out there.

Seen from the mainland, the island doesn't seem especially far – short enough that you might contemplate swimming if you were unlucky with the boat times. From the island, however, it looks different. The other shore gives you nothing to aim for, just a wall of trees and buildings in the distance.

I was the last one to reach the headland. Jumpers and T-shirts, boots and trousers lay on the rock. People often swam here, probably because the trees shielded the place; now, those same bushes helped us hide as we fled for our lives. Still, as the last one to reach the rock, it was hard to get away from the thought that the gunman might be standing there at any moment. The thick foliage would make it impossible to see him before it was too late. My boots were tied with coiled laces, attached to loops on my trousers. To untie them was out of the question. I fished my mobile out of my pocket and wrapped it in an AUF jumper that had been left on the rock. Then I took my first steps into the water, fully dressed.

Farther out, a boy was talking on his phone. He had undressed right down to his underpants and was wading out, with the water reaching his hips. With his mobile still to his ear, he started swimming and then, after a couple of strokes, he did something that surprised me. He stretched out his arm and threw the phone away. It made a splash as it landed in the water a few metres away. Seeing this, seconds after I had carefully wrapped up my own phone, felt absurd. Why on Earth?

When the ice-cold water crept up the hollow of my neck, when my boots and trousers became as heavy as lead, it wasn't so bad that it stopped me from keeping up with the others. But the feeling that we had overreacted – that the whole swim was unnecessary – bothered me. After a good 50m, my body said stop. Filled with adrenaline, it had not noticed how tired it was, but thinking of the distance we had covered and how fast we had been running, I suppose it was only a question of time. My throat tied itself up, making it almost impossible to breathe. My arms and legs were slowly but surely losing strength. With every stroke, it became more difficult to stay afloat. I tried to turn on to my back, but that was just as hard. In normal conditions, I am an above-average swimmer, but the usual rules no longer applied. It was as if something was gripping my ankles and pulling me down into the depths. I tried to undo my laces, but every time I lowered my arms, I went under and swallowed water. I was going to sink before I could take off those boots.

It was becoming increasingly difficult to think clearly. One voice was telling me to follow the others, that I was in danger. Another was saying this was an overreaction that could cost me my life. But eventually there was only one option in my head: to turn back. If this was an exercise, I certainly didn't want to drown. The group in front was almost out of sight. Another group was on its way out from the island. One of them, a girl, swam over to ask if I was OK. When she realised that I was on my way back, she tried to stop me, but she only ended up pulling me under.

"Stop it," I said. "I'm swimming back."

The last 40m back to the headland were a desperate struggle to keep my head above water. No matter how much I fought with my arms and legs, it wasn't enough. I can't have been more than 20m from the shore when everything stopped and I was pulled under one last time. Drowning has always seemed to me the worst imaginable way of dying. Was I going to end my life here? Right next to land? All sounds disappeared and a strange light shone below, as if the bottom were dimly glowing.

I stretched out my feet to feel about. They struck rock. It was just shallow enough that I could get my chin above the water. With what little strength I had left, I took two unsteady steps up on to the headland. I coughed. It felt as if I were bringing up blood, but it was only water. After emptying my lungs and regaining my breath, I staggered towards the rock. Only there could I be safe.

The water reached up to about my waist when I saw him coming out from behind the bushes. Calm and controlled, he stood on an outcrop barely 7m from where I was standing, his gaze locked on the swimmers. I crouched still, facing half towards him and half towards my friends in the water. My T-shirt clung to my chest. He raised what looked like an automatic weapon and rested his cheek on it. Out there, the outlines of about 10 heads could be seen on the surface of the water. Then he shot. There was a sound unlike any other I have heard: a metallic clank followed by a deep, resounding bass note. Out in the fjord, a white pillar rose up before falling back over the water, like rain. Nowhere near. He aimed and fired again. This time the shot hit somewhere different. Standing right down by the water, it must have been hard for him to aim: 1mm too high and the bullet struck 10m too far; 1mm too low and it fell 10m short. He fired another shot. Another miss.

A wounded woman is brought ashore. Several of the 69 people who died were shot by Breivik as they tried to swim away. Photograph: AFP/Getty Images

This wasn't a man who was shooting to create fear. He was shooting to kill. My friends were swimming for their lives. While I stood as still as I could in the water, I had a feeling I had seen him before. Was it in a Nazi film? His trousers were tucked into his boots in a military fashion. His hair was meticulously combed backwards. His gaze was as hard as stone.

After a couple of shots, he lowered his weapon and shouted: "I'm going to kill you all!"

It came out of nowhere. From seeming calm and indifferent, he suddenly flipped off balance. He hesitated a little, almost self-consciously, before screaming again, this time so loudly that his voice cracked. "You're going to die!" Then he raised his weapon once more and shot. How many shots could he have fired? Four? Five? I'm not sure, but after a couple of shots he struck the middle of one group of swimmers. Another column of water rose up. This time, it looked like it was another colour. When he had finished, he lowered his weapon to his hip. He stood there looking across the fjord, then he turned round.

In one long, unbroken, sweeping look, he moved his gaze from the swimmers to me and lifted his weapon. The water was halfway up my thighs. I wasn't going anywhere. He had a living target in front of him. All he had to do was squeeze the trigger. He rested his cheek on the rifle and creased his brow.

"No!" I yelled with what little strength I had left. My arms hung limply by my sides. "Don't shoot!"

I stopped breathing. My heart was beating so hard that it must have been visible through my wet T-shirt. Where was he going to hit me? In the head? In the heart? I hoped for the heart. And that it would be quick. I had never had a gun aimed at me before. It was a feeling of complete inferiority. He could do to me whatever he wanted. Dead or alive. It was up to him. I searched for a response from the man behind the weapon, but all I could see was the open black chasm in his telescopic sight. My body prepared itself for the bullet that was going to penetrate it. My skin stung around my heart and on my forehead.

He closed his eye.

Now it was over.

Then he lowered his weapon, turned on his heel and vanished. I gasped for air. My knees gave way beneath me. With shaking steps, I barely managed to get up on to the island before I collapsed on the rock. For some reason, I was still alive.

• This is an edited extract from Heart Against Stone: The Story Of A Survivor From Utøya, by Adrian Pracon and Erik Møller Solheim, published in Norway by Cappelen Damm. Translated by Guy Puzey.