Commuters fled in panic after the device sent a "fireball" and a "wall of flame" through the packed London Underground train.

The explosion is being treated as a terrorist incident, Scotland Yard said.

ISIS has since claimed responsiblity for the attack with Prime Mininster Theresa May upping the UK's terror threat level to critical.

The blast appears to have originated from a white bucket in a Lidl bag – which appears to have wires protruding from it – found at the rear of the carriage.

It is reported the device was only a "partial detonation" and police confirmed it was an "improvised explosive device".

Eyewitnesses saw several people "covered in blood" after a "flash and a bang" on the District Line service.

A total of 29 people were taken to hospital, with reports of "facial burns" – but London Ambulance Service confirmed "none have life threatening injuries".

Emergency services including armed police rushed to the scene at Parsons Green station after being alerted at around 8.20am on Friday.

(Image: TWITTER)

Bomb squad officers, armed police, paramedics and firefighters have been in attenance at the incidence – the cause and motive of which remain unknown.

Mrs May chaired a meeting of the Government's Cobra emergencies committee to discuss the incident.

There were also unconfirmed reports of second device and a knifeman on the run amid the chaos.

People were hurt in a stampede to exit the station following the blast, which comes after Britain has suffered under a wave of terrorist attacks.

(Image: TWITTER)

(Image: Sylvain Pennic)

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.

Mayor of London Sadiq Khan has appealed for calm, saying London "will never be intimidated or defeated by terrorism".

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."

(Image: DS) (Image: TWITTER)

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.”

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.”

(Image: REUTERS) (Image: TWITTER)

London Fire Brigade said six fire engines, a fire rescue unit and around 50 firefighters and specialist officers were sent to the scene.

Foreign Secretary Boris Johnson said: "I'm afraid my information is limited and it really is important not to speculate at the moment.

"Obviously, everybody should keep calm and go about their lives in a normal way, as normal as they possibly can.

"As far as I understand it, the British Transport Police and TfL are on it and they will be updating their websites as and when we have more information."