An “improvised explosive device” was detonated on a Tube train in south-west London during Friday’s rush hour, Scotland Yard has confirmed.
The blast, at Parsons Green station on an eastbound District Line train from Wimbledon, is being treated as terrorism.
Twenty-two people are being treated in hospital, mostly for burn injuries.
A hunt for the person who placed the device is under way, with hundreds of detectives and MI5 investigating.
The area surrounding Parsons Green Tube station has been evacuated while specialist officers secure the remains of the improvised device and ensure 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.
Assistant Commissioner Mark Rowley earlier refused to reveal whether anyone had yet 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.
Our 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.
US President Donald Trump tweeted that “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.
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.
Mayor of London Sadiq Khan earlier appealed for calm, saying the city “will never be intimidated or defeated by terrorism”.
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.
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.”
Passenger 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.