An “improvised explosive device” was detonated on a Tube train in south-west London during Friday’s rush hour, Scotland Yard has confirmed.
The blast, at Parsons Green station on an eastbound District Line train from Wimbledon, is being treated as terrorism, police said.
Twenty-two people are being treated in hospital, mostly for burn injuries.
Assistant Commissioner Mark Rowley said hundreds of detectives, working with MI5, are investigating the explosion.
Mr Rowley refused to answer whether anyone had yet been arrested over the incident at the station.
Pictures show a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor.
The BBC understands the device had a timer.
“At Parsons Green Tube station there was an explosion on a Tube train,” Mr Rowley said.
“This was a detonation of an improvised explosive device. The scene currently remains cordoned off and the investigation continues…
“Londoners can expect to see an enhanced police presence, particularly across the transport system across the day.”
He asked the public to remain “vigilant” and said they should “not be alarmed”.
Mr Rowley said anyone who took pictures or videos at the scene can upload them to ukpoliceimageappeal.co.uk.
Prime Minister Theresa May tweeted: “My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident.”
The government’s emergency committee, Cobra, is meeting at 13:00 BST.
Home Secretary Amber Rudd tweeted: “Grateful for the swift response from emergency services at Parsons Green, my thoughts are with the victims of this terrorist incident.”
Mayor of London Sadiq Khan has appealed for calm, saying the city “will never be intimidated or defeated by terrorism”.
US President Donald Trump tweeted: “Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!”
NHS England said 22 patients were being treated at Imperial, Chelsea and Westminster, Guy’s and St Thomas’ and the Central London Community Healthcare Urgent Care Centre.
Eighteen people were taken by ambulance and a further four arrived at hospital themselves.
Witnesses have described seeing at least one passenger with facial injuries, while others spoke of “panic” as alarmed passengers left the train at the station, which is above ground.
Passenger Peter Crowley said he was sitting in the carriage, travelling from Wimbledon, when the explosion happened.
He said his head was burned by a “really hot intense fireball above my head” and added: “There were people a lot worse than me.”
Passenger Chris Wildish told BBC Radio 5 live he saw a bucket in a supermarket bag with “low-level flames coming out of it” by the door of the rear carriage.
Source: BBC