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.
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.
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 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.
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".
"Incident". Incident? It was an ATTACK. At the very least, an attempted attack. I'd say that something that was supposed to go BOOOOOM went 'FOOM! FSSSSST' instead, causing some fire and burns, panic, etc, but not lots of deaths and maimings. Perhaps some jihadis cooking up explosives didn't quite manage to understand their recipe. And let us all observe that this ATTACK - not 'incident' - was carried out on a FRIDAY, the favourite Mohammedan day for murder and mayhem and human sacrifices to 'allah'.