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 toParsons 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."
Chris Wildish, who was on the train, said he saw a "device" in the last carriage.
"It was a white bucket, a builder's bucket," he told BBC 5 live.
"Flames were still coming out of it when I saw it and had a lot of wires hanging out of it - I can only assume it was done on purpose.
"It was standing against the door of the rear-most carriage."
Media technology consultant Richard Aylmer-Hall, 53, was sitting on the service bound for central London when panic unfolded at around 8.20am.
He said he saw several people injured, having apparently been trampled as they tried to escape.
He told the Press Association: "I was blissfully reading my paper and listening to a podcast and suddenly the whole world charged past me down the platform, down the Tube.
"I was on the Tube, we had just stopped at Parsons Green, I was on my way up from Wimbledon towards Paddington and 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.
"It was an absolutely packed, rush-hour District Line train from Wimbledon to Edgware Road.
"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.
"Some people got pushed over and trampled on, I saw two women being treated by ambulance crews.
"Since then, every emergency vehicle in London has gone past me, fire engines, ambulances, every type of police vehicle."
Mr Aylmer-Hall added: "I don't think anyone was hurt by the actual device or whatever it was."
One man who arrived at the station in the aftermath of the incident told the BBC: "I was walking towards the station and suddenly everyone came running down the stairs and out of the door.
"Lots of people shocked and screaming and saying there had been an explosion and people were saying there had been a wall of flame coming down the Tube carriage, I turned round and came back from the station with all of them.
"A wall of flame coming down the carriage after a bang."
One woman standing on the platform as passengers left the Tube carriage in footage of the aftermath was heard saying: "We could see it on the far end of the train, I don't know what it was - it looked like a fireball.
"Something went off - I could see it.
"I saw the thing and everybody panicked and got crushed."
BBC correspondent Riz Lateef, who was at the station travelling 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."
One passenger, named only as Lucas, told BBC 5 live: "I heard a really loud explosion - when I looked back there appeared to be a bag but I don't know if it's associated with it.
"I saw people with minor injuries, burnings to the face, arms, legs, multiple casualties in that way.
"People were helping each other."
Commuter Robyn Frost was arriving at the station when she saw people trying to escape.
She told the BBC: "I walked into the station, there was blood on the floor and people running down the stairs screaming 'get out'.
"People were coming out of the station covered in blood."
Passengers on the train behind the affected Tube witnessed terrified passengers sprinting away up the tracks.
They were then held in their train for over an hour before being evacuated on to the tracks themselves.
Nicole Linnell, 29, who works for a fashion label, said: "We saw people running down the tracks. About 30 or 40 people.
"They were running down the tracks outside our train.
"It was absolutely terrifying. Running on the tracks is the last thing you want to do so we were like 'What's going on?'
"After about an hour we were evacuated off the train on to the tracks. About 10 to 15 people at a time."
A major incident has been declared at St Mary's Hospital in Paddington in response to theParsons Green explosion.
Ryan Barnett was caught up in the crush that took place in the stairwell of the Tube station.
He heard of victims with burn injuries and reported seeing some commuters taking selfies on the platform.
The 25-year-old, who works in politics and had been travelling to his office, told the Press Association: "I was lucky, I wasn't in the carriage, I was in a few carriages down but I was caught up in the stampede down the stairs.
"I was sitting there, headphones in, at Parsons Green, the doors open fine, I'm not really paying attention and all of a sudden hundreds of people run past me screaming a mixture of 'stampede', 'attack', 'terrorist', 'explosion', 'get off the train', 'everyone run'.
"Parsons Green station has a staircase at the front of the carriage where the train pulls up - I'm running and keeping my head down because there might be gunfire and on to the staircase.
"But when I was on the staircase, the stewards and other passengers, they are shouting 'stop, stop, stop', so I ended up squashed on the staircase, people were falling over, people fainting crying, there were little kids clinging on to the back of me.
"It is absolute chaos, it was quite scary because at one stage we thought we might be trapped there - I heard a pregnant woman lost her shoes and had fallen over."
Ferried back on to the station platform, Mr Barnett began hearing from others around him about the state of those injured.
He said: "Someone said there had been an explosion, there was white stuff, someone said there was a lot of wires, one girl outside the station had her leg pretty badly cut up.
"People were saying there was one man who was singed down the side of his body, including his hair.
"It felt like an eternity, but it was only a couple of minutes, probably.
"There were a lot of people taking photos, there was a couple of people taking selfies and I thought: 'What is the world coming to?'"
The Metropolitan Police urged anyone with photographs from the scene to upload them to www.ukpoliceimageappeal.co.uk to aid the investigation.