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 and the area around the station has been evacuated.
Specialist officers there are 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 the MI5 are investigating the attack.
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.
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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 speak of ‘panic’
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.
By BBC home affairs correspondent Dominic Casciani
This is the fifth terrorism incident of 2017 in which an attack has taken place. It’s the only one this year in which nobody has died. The previous four saw 36 people killed.
Police believe they have stopped six other significant plots – all of which will soon be coming before the courts.
Put plainly, this is the most sustained period of terrorist activity in England since the IRA bombing campaign of the early 1970s.
What happens now? The Metropolitan Police’s Counter-Terrorism Command needs to get to the bottom of three key questions. One: Who did it? Two: Are there any more devices out there? Three: Who else, if anyone, is involved?
Experts from the government’s secret explosives research laboratory will be looking at the evidence from the train and seeing whether it matches anything else they have seen before.
One witness, called Luke, told 5 live there was “a sort of loud explosion”.
“It happened just as we were pulling up to the Tube station so everyone just sort of piled out of the Tube and there was a distinct smell of burning,” he said.
Emma Stevie, 27, who was on the train when the explosion happened, said she was caught in a “human stampede” and crush on the station steps as people rushed away from the train.
“I wedged myself in next to a railing, I put myself in the foetal position,” she said.
“There was a pregnant woman underneath me, and I was trying really hard not to crush her.
“I saw a poor little boy with a smashed-in head and other injuries. It was horrible.”
At Parsons Green
By Jennifer Scott, BBC News
London has again fallen victim to a terror attack, targeting commuters on their way to work and visitors venturing out into the city.
With 22 people hospitalised, this bustling part of zone two was left confused and shocked, waiting anxiously for an explanation.
Olaniyi Shokumai was riding the Tube on the way to a training course, with his headphones plugged in and listening to music – just as he would on any other journey.
After the train pulled into Parsons Green station, however, things became far from ordinary.
“I looked to my left and I saw lots of people running out. There were a lot of people just stamping on each other and I saw someone jumping over [them].
“There was a woman on the floor and she couldn’t breathe. I thought, ‘This has got to be serious for people to do that.'”
BBC London presenter Riz Lateef, who was at Parsons Green on her way in to work, said: “There was panic as people rushed from the train, hearing what appeared to be an explosion.
“People were left with cuts and grazes from trying to flee the scene. There was lots of panic.”
BBC News presenter Sophie Raworth said she saw a woman on a stretcher with burns to her face and legs.
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