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.
BBC security correspondent Frank Gardner said the incident could have been “far worse” as indications were that the bomb “may have partially failed”.