Several people have been injured at Parsons Green underground station in west London after an explosion which officials are treating as a terrorist incident.
British Transport Police (BTP) have declared it a 'major incident'. Scotland Yard's counter-terrorism command, S0 15, has taken over the lead in the investigation from BTP.
Passengers reported seeing a device in a 'bucket' on a tube train at the station and hearing a bang during the Friday morning rush-hour.
Downing Street said that the Prime Minister, Theresa May, was 'receiving regular updates' on the situation.
Armed officers attended the scene this morning, along with the London ambulance service and the London fire brigade.
A Transport for London spokesperson said that the District line had been suspended between Earl's Court and Wimbledon due to a security alert.
One witness said that there had been a 'stampede' of passengers.
Media technology consultant Richard Aylmer-Hall, 53, was sitting on the District line train bound for central London at about 8.20am when he said there was 'panic' and 'screaming'.
He said: '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.'
He added: '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.'
A reporter from the Metro newspaper said he had seen people with facial burns, while the BBC's Sophie Raworth said she had seen two ambulances leaving the scene and several people treated, including a woman with burns.
Chris Wildish, who was on the train which was targeted, described the device to BBC Radio Five Live: 'It was a white bucket, a builder's bucket, in a white Aldi bag or Lidl bag...Flames were still coming out of it when I saw it and had a lot of wires hanging out of it – I can only assume it was done on purpose,. It was standing against the door of the rear-most carriage.'
A 100-metre cordon was erected around the station, and at about 9.40am five Metropolitan police vans arrived on the scene along with a BTP command van.
Outside the station, a woman was sitting on a pavement with a bandage around her leg, while armed police patrolled. A Reuters witness saw a woman being carried off on a stretcher with her legs covered in a foil blanket.
'We are aware of an incident at #ParsonsGreen tube station. Officers are in attendance,' London police said on Twitter. 'We would advise people to avoid the area.'
An unverified picture circulating on social media showed a white bucket in a supermarket freezer bag with what appeared to be wires coming out of the top on the floor of one train carriage.
Police said they were aware of reports on social media and said they would release facts regarding the incident once they could be sure of their accuracy.
A Reuters witness could see a bomb disposal unit at the scene while the fire brigade said it had sent six engines and 50 firefighters. London Ambulance said it had sent "multiple resources" including its hazardous area response team to the scene.
'Our initial priority is to assess the level and nature of injuries,' it said.
Britain has suffered four attacks blamed on terrorists so far this year which killed 36 people. In 2005, 52 people were killed when four British Islamists carried out suicide bomb attacks on three London underground trains and a bus.