At 04:00 the Germans cut the phone and electricity lines to the building.
At 04:45, just as the German battleship Schleswig-Holstein started shelling the nearby Polish Army military outpost at Westerplatte, the Danzig police began their assault on the building under the command of Polizeioberst Willi Bethke.
They were soon reinforced by local SA formations and the SS units SS Wachsturmbann "E" and SS Heimwehr Danzig, supported by three police ADGZ heavy armoured cars.
Albert Forster, head of the local Nazi party, arrived in one of the vehicles to watch the event.
Journalists from local newspapers, Reichssender Danzig (the state radio station), and the newsreel company Ufa-Tonwache also came to cover the battle.

The first German attack, from the front, was repelled, although some Germans managed to break through the entrance and briefly enter the building (at the cost of two killed and seven wounded attackers, including one group leader).
The second attack, from the work office, was also repelled.
The commander of the Polish defence, Konrad Guderski, died during the second attack from the blast of his own grenade, which stopped the Germans who had broken through the wall.
With the Poles putting up more determined resistance than expected, Bethke suggested that his forces blow up the building with high explosives.
Forster vetoed his proposal.

At 11:00 German units were reinforced by the Wehrmacht with two 75 mm artillery pieces and a 105 mm howitzer, but the renewed assault, even with the artillery support, was again repulsed.
Mortar support was requested from the German forces at Westerplatte, but its inaccurate fire posed a greater threat to the attackers and it soon ceased action.
At 15:00, the Germans declared a two-hour ceasefire and demanded that the Polish forces surrender, which they refused.
In the meantime, German sappers dug under the walls of the building and prepared a 600 kg explosive device.
At 17:00, the bomb was set off, collapsing part of the wall, and German forces under the cover of three artillery pieces attacked again, this time capturing most of the building except the basement.

Frustrated by the Poles' refusal to surrender, Bethke requested a rail car full of gasoline.
Danzig's fire department pumped it into the basement, and it was then ignited by a hand grenade.
After three Poles were burned alive (bringing the total Polish casualties to six killed in action), the rest decided to capitulate.
The first two people to leave the building, director Dr. Jan Michoń, carrying a white flag, and commandant Józef Wąsik, were mortally wounded by the Germans.
The rest of the Poles were allowed to surrender and leave the burning building.
Six people managed to escape from the building, although two of them were captured in the following days.
At 04:00 the Germans cut the phone and electricity lines to the building. At 04:45, just as the German battleship Schleswig-Holstein started shelling the nearby Polish Army military outpost at Westerplatte, the Danzig police began their assault on the building under the command of Polizeioberst Willi Bethke. They were soon reinforced by local SA formations and the SS units SS Wachsturmbann "E" and SS Heimwehr Danzig, supported by three police ADGZ heavy armoured cars. Albert Forster, head of the local Nazi party, arrived in one of the vehicles to watch the event. Journalists from local newspapers, Reichssender Danzig (the state radio station), and the newsreel company Ufa-Tonwache also came to cover the battle.
The first German attack, from the front, was repelled, although some Germans managed to break through the entrance and briefly enter the building (at the cost of two killed and seven wounded attackers, including one group leader). The second attack, from the work office, was also repelled. The commander of the Polish defence, Konrad Guderski, died during the second attack from the blast of his own grenade, which stopped the Germans who had broken through the wall. With the Poles putting up more determined resistance than expected, Bethke suggested that his forces blow up the building with high explosives. Forster vetoed his proposal.
At 11:00 German units were reinforced by the Wehrmacht with two 75 mm artillery pieces and a 105 mm howitzer, but the renewed assault, even with the artillery support, was again repulsed. Mortar support was requested from the German forces at Westerplatte, but its inaccurate fire posed a greater threat to the attackers and it soon ceased action. At 15:00, the Germans declared a two-hour ceasefire and demanded that the Polish forces surrender, which they refused. In the meantime, German sappers dug under the walls of the building and prepared a 600 kg explosive device. At 17:00, the bomb was set off, collapsing part of the wall, and German forces under the cover of three artillery pieces attacked again, this time capturing most of the building except the basement.
Frustrated by the Poles' refusal to surrender, Bethke requested a rail car full of gasoline. Danzig's fire department pumped it into the basement, and it was then ignited by a hand grenade. After three Poles were burned alive (bringing the total Polish casualties to six killed in action), the rest decided to capitulate. The first two people to leave the building, director Dr. Jan Michoń, carrying a white flag, and commandant Józef Wąsik, were mortally wounded by the Germans. The rest of the Poles were allowed to surrender and leave the burning building. Six people managed to escape from the building, although two of them were captured in the following days.
Defence of the Polish Post Office in Danzig