British police say no-one has been arrested in connection with a bombing on a London underground train that left 29 people injured after it exploded at peak hour, causing a frenzied stampede.

Key points: 23 injured after reported blast and fire on rush-hour train at Parsons Green underground station

23 injured after reported blast and fire on rush-hour train at Parsons Green underground station Police confirm explosion was caused by an improvised explosive device

Police confirm explosion was caused by an improvised explosive device The incident is being treated as terrorism

Commuters reported a noise and a flash aboard the District Line train at Parsons Green station in the city's south-west of the city, followed by chaos as hundreds of people rushed to get away from danger.

Police said they were making urgent inquiries to track down who was behind the explosion.

"Counter-terrorism detectives are making fast-time inquiries to establish who was responsible for the incident at Parsons Green tube station," the Metropolitan Police tweeted.

Assistant Commissioner Mark Rowley said hundreds of detectives were looking at surveillance camera footage, carrying out forensic work and speaking to witnesses.

He also confirmed the blast was caused by the "detonation of an improvised explosive device".

Nineteen people had been taken to hospital by ambulance, mostly with "flash burns", authorities said.

Health officials said another 10 people presented themselves to hospitals, eight of whom were later discharged.

Local media had earlier reported some people had been hurt in the stampede after the incident.

An injured woman sits on the street outside Parsons Green station in London. ( Reuters: Kevin Coombs )

People 'trampled on, crying, shocked'

Fitness instructor Olaniyi Shokunbi was onboard the train when the device exploded.

He said it was fortunate the train was still at the station when the device detonated, instead of travelling through a tunnel.

"We are lucky, because if that train was moving, there is no escaping," he told the ABC.

Initially, he did not realise what had happened.

"I thought it might be an acid attack," he said.

Mr Shokunbi said he saw passengers getting crushed by other commuters who were trying to escape.

"Those people had serious injuries, I saw people on the floor, they kind of just crawled out of the station. Possibly broken bones, legs just hanging. You see this stuff in movie … I just can't believe it."

A commuter whose train had just left the station said there was panic after a woman on the platform saw what appeared to be an explosion.

Richard Aylmer-Hall said he saw several people injured, apparently trampled as they fled.

Sorry, this video has expired London police confirmed they were treating the explosion as "terrorist-related".

"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," he said.

"Some people got pushed over and trampled on — I saw two women being treated by ambulance crews."

Mr Aylmer-Hall said he did not believe anyone was hurt by the actual device.

An injured woman is helped by a police officer outside Parsons Green station. ( AP: Dominic Lipinski/PA )

Another man, Ola Fayankinnu, described flames engulfing one carriage and racing along the train, forcing passengers to trample others as they rushed for an exit.

"I was on [the] second carriage from the back. I just heard a kind of whoosh," he said, adding that the train was packed with people and he had not heard a bang.

"I looked up and saw the whole carriage engulfed in flames making its way towards me.

"A lot of people were trampled on. There were phones, hats, bags all over the place and when I looked back I saw a bag with flames.

"People were crying, shocked, a few people had been injured, some people had been trampled."

Sorry, this video has expired A white container inside a plastic bag burns on the London Underground train. (Photo: Twitter/Rigs)

Another witness described seeing ambulances, fire engines and a helicopter at the station, and another saw a woman being carried off on a stretcher with her legs covered in a foil blanket.

Assistant director of operations at London Ambulance Service Natasha Wills said in a statement on Twitter that a hazardous area response team had been sent to the scene.

"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," she said.

"Our initial priority is to assess the level and nature of injuries."

May to chair emergency response committee meeting

The mayor of London, Sadiq Khan, said the city would "never be intimidated or defeated by terrorism".

Mr Khan said London "utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life".

Foreign Secretary Boris Johnson told the BBC: "Everybody should keep calm and go about their lives in as normal a way as they possibly can."

Members of a bomb disposal squad were also on the scene of the incident. ( Reuters: Hannah McKay )

Prime Minister Theresa May said Britain's official threat level from terrorism would remain at "severe", meaning an attack is highly likely.

After chairing a meeting of the Government's emergency committee, which Mr Khan also attended, Ms May said the threat level was not being raised to "critical", which would mean an attack is imminent.

In an earlier statement, she 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."

US President Donald Trump responded on Twitter, calling the perpetrator a "loser terrorist".

"Loser terrorists must be dealt with in a much tougher manner," he said.

"The internet is their main recruitment tool which we must cut off and use better!"

Asked about Mr Trump's comments, Ms May said: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."

Police officers direct people in the area after the incident. ( Reuters: Kevin Coombs )

ABC/Reuters/AP