An explosion on a Tube train in south-west London is being treated as terrorism, Scotland Yard has said.
Passengers were injured following the blast at 08:20 BST (07:20 GMT) at Parsons Green District Line station.
Pictures show a white bucket on fire inside a supermarket bag, but do not appear to show extensive damage to the inside of the carriage.
The Metropolitan Police said “a number of people” had been injured and advised people to avoid the area.
It was too early to confirm the cause of the fire and the station has been cordoned off, the statement said.
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”.
Witnesses have described seeing at least one passenger with facial injuries.
Others have spoken of “panic” as alarmed passengers left the train at Parsons Green station.
Passenger Peter Crowley said he was sat in the carriage, travelling eastbound 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.
“I certainly saw some burning injuries,” he said, but added that “everyone behaved in the right manner as such, everyone got off as quickly as they can and you know supported everyone”.
Emma Stevie, 27, who was at Parsons Green station, said: “We were running down the stairs like… it felt like for our lives.
“I went down the stairs and after a while people were just piling on top of each other, because people were falling over trying to run so quickly.
“There were two ladies underneath me and a little boy to my right, his head had been smacked into the concrete.”
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.
Media technology consultant Richard Aylmer-Hall who was sitting on the “packed” District Line train said he saw several people injured, having apparently been trampled as they tried to escape.
The 53-year-old said: “Suddenly there was panic, lots of people shouting, screaming, lots of screaming.
“There was a woman on the platform who said she had seen a bag, a flash and a bang, so obviously something had gone off.
“I saw crying women, there was lots of shouting and screaming, there was a bit of a crush on the stairs going down to the streets.”
Natasha Wills, assistant director of operations at London Ambulance Service, said: “We were called at 8:20 to reports of an incident at Parsons Green underground station.
“We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team, with the first of our medics arriving in under five minutes.”
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