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 station in Fulham.
Pictures show a white bucket on fire inside a supermarket bag, but do not appear to show extensive damage to the inside of the Tube train carriage.
The Metropolitan Police said it was too early to confirm the cause of the fire and the station has been cordoned off.
BBC security correspondent Frank Gardner said it was too early to say who caused the explosion.
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.
London Ambulance Service says it sent a hazardous area response team to the scene.
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.
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, 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 says she saw a woman on a stretcher with burns to her face and legs.
Alex Littlefield, 24, a City worker, said: “I was walking around the corner to the Parsons Green Tube station and I saw the raised platform with everyone running and looking upset.
“I saw police officers, fire brigade… masses of people and armed police. There were lots of very, very distressed people. We’ve been pushed right back now.”
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.
“Our initial priority is to assess the level and nature of injuries. More information will follow when we have it.”