Emergency services were called to a London Underground station after reports of an explosion on a train.
Pictures appeared to show an explosion in a bucket towards the rear of a train.
The incident occurred at Parsons Green Tube station on Friday morning.
A metro.co.uk reporter said passengers suffered facial burns.
The Metropolitan Police said: "We are aware of an incident atParsons Green Tube station. Officers are in attendance."
British Transport Police said: "We're aware of an incident at Parsons Green station. Officers are at the scene. The station is closed."
Transport for London said it is "investigating an incident at Parsons Green" and urged customers travelling between Wimbledon and Earls Court to use alternative routes.
A London Ambulance Service spokesman said: "We are on scene alongside the Metropolitan Police at Parsons Green."
London Fire Brigade said they were called to the incident at 8.21am on Friday.
Media technology consultant Richard Aylmer-Hall, 53, was sitting on the District Line train 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."
Natasha Wills, of London Ambulance Service said: "We were called at 8.20am to reports of an incident at Parsons Green underground station.
"We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team, with the first of our medics arriving in under five minutes.
"Our initial priority is to assess the level and nature of injuries. More information will follow when we have it."
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."
Transport for London said Tube services were suspended between Edgware Road and Wimbledon.
In a statement, Scotland Yard said: "Police were called at approximately 8.20am 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."
Another witness, Sham, said he saw a man with blood all over his face.
"There were loads of people crying and shaking," he told 5 live.
"There were a lot of people limping and covered in blood.
"One guy I saw, his face was covered in blood - I've never seen anything like it."
London Fire Brigade said six fire engines, a fire rescue unit and around 50 firefighters and specialist officers were sent to the scene.
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."
She added: "People have been stretchered into ambulances now."
BBC News presenter Sophie Raworth, who was at the scene, said she saw a woman on a stretcher with burns to her face and legs.
"I spoke to a woman, Laura Bishop, who said she was standing there and all of a sudden heard a bang - and saw a wall of flames, flames in the carriage," she said.
"She saw people who had burns on their hair, burns on their head and the doors opened immediately."