Commuters fled in terror after a blast sent a "fireball" and a "wall of flame" through a packed London Underground train.
Eyewitnesses reported seeing several people hurt and "covered in blood" after a "flash and a bang" on the District Line Tube in West London.
Emergency services including armed police rushed to the scene at Parsons Green station after being alerted at around 8.20am on Friday.
Pictures posted on social media appeared to show a flaming bucket inside a plastic carrier bag on the floor of a carriage.
Police have not given details about the suspected cause of the explosion but the incident comes with Britain on high alert after four terror attacks in 2017.
The official threat level currently stands at severe - meaning an attack is seen as "highly likely".
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."
Scotland Yard said: "Police were called at approximately 08:20hrs on Friday, 15 September to Parsons Green Underground Station following reports of an incident on a Tube train.
"Officers from the Metropolitan Police Service and British Transport Police are in attendance along with the London Fire Brigade and London Ambulance Service.
"We would advise people to avoid the area."
Mick Cash, general secretary of the Rail, Maritime and Transport union, said: "The incident shows once again that Tube staff are the first responders in emergency situations and the clear need for safety and security to remain the number one priority and for the resources and staffing to be in place to deal with all eventualities."