An "improvised explosive device" was detonated on a Tube train in south-west London during Friday's morning rush hour.
The blast, at Parsons Green station on a District Line train from Wimbledon, is being treated as terrorism.
Twenty-two people have been treated in hospitals, mostly for burns, though at least eight have now been discharged.
A hunt for the person who placed the device is under way and the area around the station has been evacuated.
Specialist officers have been securing the remains of the improvised device and ensuring it is stable.
Prime Minister Theresa May condemned the "cowardly" attack, which she said had "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.
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.