An explosion at Parsons Green underground station in the west of the capital injured passengers and led to a "major incident" being declared. Yet more people were hurt during the crush to leave the train, witnesses said.
Images shared online appeared to showed a bucket with wires trailing from it smouldering inside a District Line train.
The device is being forensically examined while detectives are scouring CCTV and examining witness statements for potential leads.
Armed police, paramedics and firefighters were all called to the scene.
Witnesses described flames and a "fireball" following the incident at about 8.20am today.
Assistant Commander Mark Rowley, who is the UK's top counter-terrorism officer, said the force continues to "chase down suspects" in connection with the attack.
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Good morning and welcome to our live blog with the latest updates on an incident at Parsons Green Tube station, where witnesses say there has been an explosion on a train. Police have not confirmed the cause of the alarm, which has caused the station and part of the District Line to be shut.
‘Many injured’ as stampede of people flee from ‘explosion’ at Parsons Green Tube stationThe IndependentPeople have been injured in a "stampede" as they attempted to escape a reported explosion on the London Underground, according to a witness. While the explosion itself may not have injured anyone, a number of people were hurt as people ran from it, one commuter said.
It's not clear yet whether anyone was injured in the blast. But witnesses report that in the panic and rush to leave the train, some people may have been injured.
The BBC's Sophie Raworth says that she saw a woman taken into an ambulance with burns "from top to toe" after the explosion.
A witness tells LBC that there are "around 20 people with physical injuries" after the explosion.
This image appears to show wires coming from the bucket that was reportedly left on the Tube, and the source of the explosion.
The video below appears to show the aftermath of the explosion, with the Tube train parked at the station and the fire still blazing.
Flames engulfed one carriage and raced along a train on a west London route to Parsons Green, forcing passengers to trample others as they rushed for an exit, an eyewitness told Reuters.
The man said people were trampled on when they fled the train after hearing a whoosh and seeing flames race towards them. He said he did not hear a bang after police rushed to an incident at the station at Parsons Green.
"I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me," he told Reuters, adding that the train was packed with people.
Police have told people to "avoid the area" after the explosion in west London.
"Officers from the Metropolitan Police Service and British Transport Police are in attendance along with the London Fire Brigade and London Ambulance Service," London's police said in a statement.
"We would advise people to avoid the area.
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