An “improvised explosive device” was detonated on a Tube train in south-west London during Friday’s morning rush hour, Scotland Yard has confirmed.
The blast, at Parsons Green station on a District Line train from Wimbledon, is being treated as terrorism.
Twenty-two people have been treated in hospitals, mostly for burns, though at least eight have now been discharged.
A hunt for the person who placed the device is under way and the area around the station has been evacuated.
Specialist officers there are securing the remains of the improvised device and ensuring it is stable.
Prime Minister Theresa May condemned the “cowardly” attack, which she said had “intended to cause significant harm”.
She said the UK’s terror threat level would remain at severe – the second highest – but would be under review.
Speaking in Downing Street after chairing a meeting of the Cobra emergency committee, she said there would be an increased armed police presence on the transport network in London.
Hundreds of detectives and the MI5 are investigating the attack, which took place at 08:20 BST on an eastbound train.
Assistant Commissioner Mark Rowley earlier refused to reveal whether anyone had yet been arrested.
St Thomas’ Hospital in London said it had treated eight patients, but they had now been discharged.
Chelsea and Westminster Hospital received 14 people, with a “small number” of them being taken to its specialist burns unit.
US President Donald Trump tweeted that “sick and demented” people behind the attack had been in the sights of the Metropolitan Police, prompting Mrs May to say it was not helpful to “speculate” on an ongoing investigation.
Mr Rowley asked the public to remain “vigilant”, but said they should “not be alarmed”.
He said anyone who took pictures or videos at the scene could upload them to ukpoliceimageappeal.co.uk.
Mayor of London Sadiq Khan earlier appealed for calm, saying the city “will never be intimidated or defeated by terrorism”.