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, BBC reported.
Twenty-two people are being treated in hospital, mostly for burn injuries.
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.
Assistant Commissioner Mark Rowley earlier refused to reveal whether anyone had yet been arrested.