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, were investigating the explosion.
A manhunt for the person who placed the device is under way.
Mr Rowley refused to reveal whether anyone had yet been arrested over the incident.
Pictures show a white bucket on fire inside a supermarket bag, with wires trailing on to the train carriage floor.
The BBC understands the device had a timer.
BBC security correspondent Frank Gardner said the incident could have been “far worse” as indications were that the bomb “may have partially failed”.
He said intelligence services would be reviewing whether to raise the national terrorism level from severe – the second highest – to critical, the highest threat, which means an attack is expected imminently.
“At Parsons Green Tube station there was an explosion on a Tube train,” Mr Rowley said.
“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”.
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 currently meeting.
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.” -BBC
