An “improvised explosive device” was detonated on a Tube train in south-west London during Friday’s rush hour, Scotland Yard has confirmed.
Assistant Commissioner Mark Rowley said hundreds of detectives, working with MI5, are investigating the blast at 08:20 BST at Parsons Green station.
Twenty-two people are being treated in hospital, mostly for burn injuries.
The blast, on an eastbound District Line train from Wimbledon, is being treated as terrorism, police said.
The Metropolitan Police said it was too early to confirm the cause of the fire and the station, which is above ground, has been cordoned off.
Pictures show a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor.
“At Parsons Green Tube station there was an explosion on a Tube train,” Mr Rowley said.
“This was a detonation of an improvised explosive device. The scene currently remains cordoned off and the investigation continues…
“Londoners can expect to see an enhanced police presence, particularly across the transport system across the day.”
He asked the public to remain “vigilant” and said they should “not be alarmed”.
Mr Rowley said anyone who took pictures or videos at the scene can upload them to ukpoliceimageappeal.co.uk.
Prime Minister Theresa May tweeted: “My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident.”
The government’s emergency committee, Cobra, is meeting at 13:00 BST.
Mayor of London Sadiq Khan has appealed for calm, saying the city “will never be intimidated or defeated by terrorism”.
US President Donald Trump tweeted: “Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!”
Witnesses have described seeing at least one passenger with facial injuries, while others spoke of “panic” as alarmed passengers left the train at the station.
Passenger Peter Crowley said he 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.
At Parsons Green
By Jennifer Scott, BBC News
A helicopter flies overhead as confused and concerned members of the community wait on the street for more news.
A police cordon has been put up around Parsons Green station, with every side street blocked off by blue and white tape.
Police vehicles are still rushing down the west London road, with sirens blazing as they pass supermarkets, furniture stores and music shops.
And bit by bit, commuters who were stuck on the trains behind are being allowed to exit.
Justine Daniels had just arrived from South Africa, laden with suitcases, when her Tube train came to a stop.
“We were on there for over an hour before they let us out through the emergency exit and we walked along the tracks,” she says.
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, and 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.
“The injuries from the stampede seemed the worst. I’m outside now, there are women crying and people sitting on the floor.”
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.
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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