More than 100 families were evacuated from a tower block of flats in Barking during the night when police said a man threatened to burn down the building.
The families were moved out of the 12-storey Earlsdown House in Wheelers Cross, on Barking's Gascoigne housing estate, at 8.40pm following threats to set it alight.
The man barricaded himself inside a flat and neighbours were evacuated as a precaution, police said.
Officers forced their way into a flat soon after 1am today and detained ther occupant, Scotland Yard confirmed.
The Met's Terrotiorial Support Group got into the flat after striking the door seven times, according to one neighbour.
A man was arrested at the flat on suspicion of criminal damage and intent to endanger life and has been taken to hospital as a precaution concerning "a pre-existing medical condition".
Emergency crews from Barking fire station were at the scene on standby as King Edward Road and the streets around Earlsdown House were cordoned off.
"Our role was a precaution on standby because of threat of fire," London Fire Brigade watch manager Daren Horrigan told the Barking and Dagenham Post.
"We were there to assist police who organised the evacuations from the building, to be on hand if the worst came to the worst and there was a fire."
Council officials arrived to arrange overnight emergency accommodation for the families being evacuated. But the families were later allowed back to their homes following the man's arrest.
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