– ‘Wall of fire’ –
May announced on Friday that 1,000 troops would be deployed to take on the responsibility for guarding key sites, including nuclear facilities.
The critical alert terror warning was last used after the deadly suicide attack at Manchester Arena, also claimed by IS, in May.
Witnesses described chaos after Friday’s explosion.
Charlie Craven told AFP he heard a “massive bang” and “an orange sort of fireball encompassing the whole Tube coming towards you.”
Another, Lauren Hubbard, described it as “a wall of fire”.
Twitter user @Rrigs posted pictures of a white bucket smouldering on the train and described how a “fireball flew down carriage and we just jumped out open door”.
The bucket, which was inside a frozen food bag, looked like the type used by builders and there appeared to be cables coming from it.
– ‘I could smell the burning’ –
The remnants of the bomb were examined by forensic scientists but no further details were released. British media reported that it had a timer which had failed to properly detonate.
Several victims were taken to hospital, though health authorities said none were in a serious life-threatening condition.
Only three remained in care on Saturday morning.
Louis Hather, 21, had been travelling to work and was three carriages down from where the explosion took place.
“I could smell the burning. Like when you burn plastic,” he told AFP.
He was trampled on as panicking passengers stampeded out of the station and his leg was badly cut and bruised.
Four previous attacks in London and Manchester this year claimed the lives of 35 people.
Three of those attacks involved a vehicle ploughing into pedestrians.