Media playback is unsupported on your device Media caption Footage shows a burning bag on board the Tube carriage at Parsons Green station 
 
 An "improvised explosive device" was detonated on a Tube train in south-west London during Friday's morning rush hour, injuring 29 people. 
 
 The blast, at Parsons Green station on a District Line train from Wimbledon, is being treated as terrorism. 
 
 So-called Islamic State says it carried out the attack, which Prime Minister Theresa May condemned as "cowardly". 
 
 A hunt is under way for the person who placed the device and the area around the station has been evacuated. 
 
 Specialist officers there securing the remains of the improvised device and ensuring it is stable. 
 
 'Cowardly attack' 
 
 Mrs May said the attack had been "intended to cause significant harm". 
 
 She said the UK's terror threat level would remain at severe - the second highest - but would be under review. 
 
 Speaking in Downing Street after chairing a meeting of the Cobra emergency committee, she said there would be an increased armed police presence on the transport network in London. 
 
 Hundreds of detectives and MI5 are investigating the attack, which took place at 08:20 BST on an eastbound train. 
 
 Assistant Commissioner Mark Rowley earlier refused to say whether anyone had been arrested. 
 
 Image copyright PA Image caption One commuter took a picture of the device, which appeared to have wires protruding from a white canister 
 
 Pictures taken of the train show a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor. The BBC understands the device had a timer. 
 
 BBC security correspondent Frank Gardner said the bomb appeared not to have gone off. 
 
 Had it worked as intended, it would have killed everyone around it and maimed everyone in the train carriage for life, he said. 
 
 Media playback is unsupported on your device Media caption PM: Trump speculation "not helpful" 
 
 US President Donald Trump has spoken to Prime Minister Theresa May to convey his sympathies for those injured in the terrorist attack, the White House has said. 
 
 President Trump pledged to continue close collaboration with the UK to stop attacks and combat extremism, the statement added. 
 
 A Downing Street spokesman said: "President Trump called the prime minister earlier today to offer his condolences over this morning's cowardly attack in London." 
 
 US President Donald Trump had earlier tweeted that the "sick and demented" people behind the attack had been in the sights of the Metropolitan Police, prompting Mrs May to say it was not helpful to "speculate" on an ongoing investigation. 
 
 Media playback is unsupported on your device Media caption Assistant Commissioner Mark Rowley said the blast was caused by an improvised explosive device 
 
 Chelsea and Westminster Hospital has been treating 14 people, with a "small number" of them being taken to its specialist burns unit. 
 
 Four people are being treated by Imperial College Healthcare and three at St George's Hospital. 
 
 St Thomas' Hospital in London said it had treated eight patients but they had now been discharged. 
 
 Mr Rowley asked the public to remain "vigilant", but said they should "not be alarmed". 
 
 He said anyone who took pictures or videos at the scene could upload them to ukpoliceimageappeal.co.uk. 
 
 London Mayor Sadiq Khan appealed for calm, saying the city would "never be intimidated or defeated by terrorism". 
 
 Witnesses speak of 'panic' 
 
 Witnesses to the incident have described seeing at least one passenger with facial injuries, while others spoke of "panic" as alarmed passengers left the train at the station, which is above ground. 
 
 Anna Gorniak, who was in the same Tube carriage as the explosion, said: "I could see a fireball filling the carriage and coming our way. At that moment, I started to run. 
 
 "In my mind I was praying, I probably thought for a second, 'That's it, my life is over.'" 
 
 Image caption Anna Gorniak said she could see a fireball "filling the carriage" 
 
 Media playback is unsupported on your device Media caption Peter Crowley: "There was a fireball above my head" 
 
 Passenger Peter Crowley was sitting in the carriage, travelling from Wimbledon, when the explosion happened. 
 
 He said his head was burned by a "really hot intense fireball above my head" and added: "There were people a lot worse than me." 
 
 Chris Wildish told BBC Radio 5 live he saw a bucket in a supermarket bag with "low-level flames coming out of it" by the door of the rear carriage. 
 
 Analysis 
 
 Image copyright AFP Image caption The blast happened at 08:20 BST at Parsons Green District Line station 
 
 By Dominic Casciani, BBC home affairs correspondent 
 
 The bomb at Parsons Green was an improvised explosive device (IED) and, like those on 21/7, it didn't work as planned. 
 
 The device's initiator appears to have worked - but the main charge did not detonate. This could be because the bomb-maker had got the recipe wrong. 
 
 In the case of the 21/7 attacks, the attackers tried to create TATP, an explosive compound made from hydrogen peroxide, commonly sold as hair bleach, and other chemicals. 
 
 They added chapatti flour to make the explosion more powerful still. But their chemistry was incompetent - in fact so incompetent that the ringleader had no idea, when he strapped on his rucksack, whether his device was capable of detonating or not. 
 
 TATP was used in the Manchester Arena attack earlier this year - and it was also the chemical of choice for some of the Paris 2015 attackers and the men behind the Brussels Airport bombing the following year. 
 
 Read more 
 
 Image copyright Getty Images Image caption There have been a "number of people" injured, police said 
 
 Media playback is unsupported on your device Media caption Witnesses describe the explosion at Parsons Green 
 
 Emma Stevie, 27, who was on the train when the explosion happened, said she was caught in a "human stampede" and crush on the station steps as people rushed away from the train. 
 
 "I wedged myself in next to a railing, I put myself in the foetal position," she said. 
 
 "There was a pregnant woman underneath me, and I was trying really hard not to crush her. 
 
 "I saw a poor little boy with a smashed-in head and other injuries. It was horrible." 
 
 Media playback is unsupported on your device Media caption BBC presenter Sophie Raworth was near Parsons Green station minutes after the incident happened 
 
 At Parsons Green 
 
 Image copyright Reuters Image caption Witnesses have described seeing at least one passenger with facial injuries 
 
 By Jennifer Scott, BBC News 
 
 London has again fallen victim to a terror attack, targeting commuters on their way to work and visitors venturing out into the city. 
 
 With 22 people treated in hospital, this bustling part of zone two was left confused and shocked, waiting anxiously for an explanation. 
 
 Olaniyi Shokumai was riding the Tube on the way to a training course, with his headphones plugged in and listening to music - just as he would on any other journey. 
 
 After the train pulled into Parsons Green station, however, things became far from ordinary. 
 
 "I looked to my left and I saw lots of people running out. There were a lot of people just stamping on each other and I saw someone jumping over [them]. 
 
 "There was a woman on the floor and she couldn't breathe. I thought, 'This has got to be serious for people to do that.'" 
 
 Ladders used to evacuate train ||||| Theresa May makes TV statement as police search for bomber who left dozens injured by blast at Parsons Green station 
 
 Britain’s terror threat level has been raised from severe to critical, indicating a further attack may be imminent following the Parsons Green tube bombing, Theresa May has said. 
 
 The hunt is under way for the bomber who attempted to bring carnage to a London rush-hour tube train packed with schoolchildren and commuters. 
 
 
 
 London tube bombing: 18-year-old man arrested in Dover - live updates Read more 
 
 Twenty-nine people including a young boy were injured when the bomb partially detonated and sent a ball of fire along a carriage of a District line train at Parsons Green, west London. 
 
 Police are understood to have found CCTV images that captured the bomber as he boarded the train with the bomb packed in a white plastic bucket inside a Lidl supermarket bag. The train is believed to have had onboard CCTV, and there are a large number of cameras covering the network. 
 
 Islamic State claimed responsibility for the blast, the militant group’s Amaq news agency said on Friday evening. 
 
 Nobody suffered life-threatening injuries in the incident, the fifth terrorist attack Britain has suffered in less than six months. Parsons Green station has since reopened. 
 
 In a televised statement on Friday night, the prime minister said the UK terror threat level was being raised to its highest rating and that armed police and members of the military would be seen on the streets in the coming days. 
 
 
 
 Play Video 0:55 Theresa May: terror threat level raised to critical after London tube blast – video 
 
 “For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public,” she said. 
 
 
 
 “The public will see more armed police on the transport network and on our streets, providing extra protection. This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses.” 
 
 The prime minister condemned Friday’s bombing as cowardly and the home secretary, Amber Rudd, denounced it as “callous and indiscriminate”. The London mayor, Sadiq Khan, described the attack as hideous and added: “We will never be defeated.” 
 
 The security minister Ben Wallace told the BBC on Saturday: “We haven’t been able at the moment to catch the bomber. The bomber is still out there – or bombers – and we have to get to the bottom of that and follow up the leads.” 
 
 The Met assistant commissioner Mark Rowley, the UK’s most senior anti-terror police officer, said police were “chasing down suspects”. He said support from the military would free up about 1,000 extra armed police officers, largely from the Civil Nuclear Constabulary and Ministry of Defence police. 
 
 
 
 A former army bomb disposal officer said it appeared the initiating charge of the device had exploded but had failed to detonate the main charge. 
 
 
 
 Initially, police considered the possibility that the bomber had been among those taken to hospital, where many were treated for flash burns. 
 
 Investigators. however, believe the device was remotely detonated, which suggested it may not have been intended as a suicide attack, and that the bomber may have left the train before the explosion. 
 
 Donald Trump’s first response to the attack – a tweet in which he claimed the “loser terrorist” had been on Scotland Yard’s radar, and that more proactive policing was required – was dismissed by a police spokesman as “unhelpful”. May also criticised the US president, saying: “I never think it is helpful for anybody to speculate on what is an ongoing investigation.” 
 
 Donald J. Trump (@realDonaldTrump) Another attack in London by a loser terrorist.These are sick and demented people who were in the sights of Scotland Yard. Must be proactive! 
 
 The explosion happened at about 8.20am on Friday while the train was stationary and the doors open. 
 
 Rory Rigney had just boarded the rear carriage of the train and was standing just feet away when the blast went off. “I thought: ‘Did someone smash something?’ It sounded like a smash, then I heard a scream and thought: ‘It’s someone having a fight.’ Then I saw the fireball coming towards me – yellow or orange. My face still feels warm. 
 
 “The doors were still open, so I jumped out and tried to get as far away as possible, got on the ground and covered myself. I wasn’t hanging around to get a better look at it.” 
 
 Chris Wildish, who was also on the train, said “a massive flash of flames” reached up to the ceiling of the train, and then the air was filled with the smell of chemicals. Many of the passengers were schoolchildren, who were knocked around by people trying to get away from the fire, he said. 
 
 
 
 Emma Stevie said people tumbled over each other as they tried to escape. “There were people on top of people. There was a pregnant lady under me shouting: ‘I’m pregnant’. There was a little boy in his red and black uniform, he must have been about eight years old, a tiny thing, with his face smashed into the steps.” 
 
 Before long, a number of people returned to the train to retrieve bags and telephones. A few took photographs of the smouldering remains of the device, which had been packed into a bucket that eyewitnesses said appeared to have held mayonnaise. A number of wires, which appeared to be attached to small lights, were also visible. 
 
 Tube staff then ordered people to leave the station. Armed police were on the scene within minutes, as were firefighters and paramedics, who began treating people with burns. 
 
 Police threw a cordon around the area and asked some residents to leave their homes. Armed police evacuated a nursery and a junior school, and a girls’ secondary school was placed in lockdown. 
 
 NHS England said 29 patients had been treated, with 21 still in hospital on Friday evening. 
 
 Responsibility for the investigation was quickly handed to Scotland Yard’s counter-terrorism command. 
 
 Emma (@EmmaStevie1) I'm safe - just had to run for my life at #ParsonsGreen station - huge stamped, lots injured. Not sure why - fire/explosion mentioned. pic.twitter.com/zRvRPWOuzA 
 
 Rowley said hundreds of detectives had been drafted in to the investigation, and he appealed to the public for information. 
 
 So far, 77 images and videos have been sent to investigators via the UK Police Image Appeal website. 
 
 Alex Littlefield (@A_Littlefield) Still unclear but very scary - extremely heavy armed police presence now #ParsonsGreen pic.twitter.com/WkodyNsfk0 
 
 On Friday night detectives were examining a mass of CCTV footage from the underground rail network, and from businesses around some District line stations. 
 
 The device was being examined by forensics experts, and may offer clues from its construction and from the chemicals used to make the explosive charge. 
 
 Counter-terrorism investigators will want to establish where the device was put together, and where the materials to make it were acquired. They will also be attempting to establish whether the bomb-making knowledge was acquired from the plethora of DIY guides on the internet, or from another person. 
 
 Facebook Twitter Pinterest A picture posted on Twitter of a burning plastic bag holding what is believed to be an improvised explosive device on a District line train. Photograph: @RRigs 
 
 Rita Katz, the director of the private intelligence company Site, said Isis claimed the bombing was the work of a “detachment” rather than simply a “soldier”, which she said implied it was a co-ordinated attack. 
 
 
 
 The terror group has frequently declared that it was behind terrorist incidents in recent years but experts have warned that its claims should be treated with caution. 
 
 The explosion at Parsons Green is the first terrorist attack in the UK this year to not result in loss of life. Two vehicle and knife attacks in London and a bombing in Manchester were blamed on Islamist-motivated terrorism. A van attack in June was blamed on an extreme rightwing motivation. 
 
 Hans Michels, a professor of safety engineering at the chemical engineering department at Imperial College London, said there appeared to be similarities with the failed attempts on 21 July 2005 to bomb the tube network. 
 
 Play Video 0:47 'People were falling on top of each other': Parsons Green witness explains the panic 
 
 “In appearance and arrangement the remnants of the device seem highly similar to those of the hydrogen peroxide-based devices of 2005. The size of the device and its containment in a plastic bucket is also the same.” 
 
 The trial of the 21 July bombers heard that their devices had failed because they had made mistakes when mixing the chemicals. 
 
 Additional reporting: Sarah Marsh, Kevin Rawlinson and Patrick Greenfield
– An explosion and fire on a London Underground train caused panic and injuries during rush hour Friday morning. Police, ambulances, and firefighters were called to the Parsons Green station in the southwest of the city after the blast, which authorities are now calling a terrorist attack, the Guardian reports. Photos and video shared on social media showed a fire in a bucket but no extensive damage to the subway car. Passenger Richard Aylmer-Hall says he saw several people injured in a stampede to escape the packed train. "Suddenly there was panic, lots of people shouting, screaming, lots of screaming," he says. Passenger Peter Crowley tells the BBC that he was sitting on the train when he felt a "really hot intense fireball above my head." He adds: "There were people a lot worse than me." Other witnesses say they saw at least one person being taken away with burn injuries. The London Ambulance Service says at least 18 people were hospitalized after the attack, none of them with life-threatening injuries, the AP reports. London Mayor Sadiq Khan urged Londoners to remain vigilant and said the city "utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life."