A woman was reportedly severely burned in an explosion at Parsons Green underground station in west London that is being treated as a terrorist incident.
BBC presenter Sophie Raworth, who was on the packed commuter train heading into central London said she saw a woman taken to ambulance with burns "from top to toe".
Witnesses described a loud bang, followed by flames and a "fireball" that erupted in the District Line Tube carriage at about 8.20am today.
The packed commuter train also had many children on board, travelling to school.
Several passengers were injured in the explosion, and witnesses said there were scenes of panic as people struggled to escape the carriage, "collapsing and pushing" each other.
Images of the aftermath of the incident appear to show a bucket with wires trailing from it smouldering inside the carriage, near doors.
Police have cordoned off a large area around Parsons Green Tube station and say they are also evacuating all homes within 100 metres.
A nearby school was "on lockdown", police told Independent reporter Lizzie Dearden, with pupils being kept in a hall.
Independent employee Aimee Caulfield who was on the train said: "It was chaotic. Everyone was running out. Trying to get out of the stairs as quickly as possible. No-one knew what was going on. Some people wanted to wait – they thought it was just a signal failure."
Another witness, named Sham, told BBC 5 Live: “There were people on the floor, all like toppling over each other just trying to get out as quickly as they could and there were loads of people crying and shaking.”
He said there were "loads of people limping and covered in blood and stuff from like where they'd fallen over and hurt themselves”.
A passenger on the train told radio station LBC that they thought as many as 20 people were injured.
Armed police, paramedics and firefighters are all on the scene.
In a statement, the Metropolitan Police said: “At present we are aware of a number of people who have suffered injuries. It is too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Met's Counter Terrorism Command. The station remains cordoned off and we are advising people to avoid the area.”