Scotland Yard has declared a terrorist incident after a blast sent a "fireball" and a "wall of flame" through a packed London Underground train.
Officers from the Metropolitan Police's counter-terrorism command have launched an investigation following the explosion in west London during the Friday morning rush hour.
The force said police were called at approximately 8.20am to Parsons Green Underground Station "following reports of a fire on the train".
The Met said Deputy Assistant Commissioner Neil Basu, the national co-ordinator for counter-terrorism policing, "has declared it a terrorist incident", adding: "At present we are aware of a number of people who have suffered injuries."
Prime Minister Theresa May is to chair a meeting of the Government's Cobra emergencies committee this afternoon to discuss the incident.
The PM said: "My thoughts are with those injured at Parsons Green and the emergency services who, once again, are responding swiftly and bravely to a suspected terrorist incident."
Commuters fled in terror after the blast and witnesses reported seeing several people hurt and "covered in blood" after a "flash and a bang" on the District Line Tube.
Emergency services including armed police rushed to the scene and cordoned off the station.
Pictures posted on social media appeared to show wires protruding from a flaming bucket inside a plastic carrier bag on the floor of a carriage.
The Met said: "It is too early to confirm the cause of the fire, which will be subject to the investigation that is now under way by the Met's Counter Terrorism Command."
Sylvain Pennec, a software developer from Southfields, near Wimbledon, was around 10 metres from the source of the explosion when fire filled the carriage.
"I heard a boom and when I looked there were flames all around," he said.
"People started to run but we were lucky to be stopping at Parsons Green as the door started to open."
He described the scene of panic as commuters struggled to escape the carriage, "collapsing and pushing" each other.
Mr Pennec stayed behind to take a closer look at what he believed was the source of the explosion.
"It looked like a bucket of mayonnaise," he said.
"I'm not sure if it was a chemical reaction or something else, but it looked home-made. I'm not an expert though."