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 today (15), is being treated as terrorism.
Twenty-two people are being treated in hospital, mostly for burn injuries.
A hunt for the person who placed the device is under way, with hundreds of detectives and MI5 investigating.
Assistant Commissioner Mark Rowley earlier refused to reveal whether anyone had yet been arrested.
Pictures taken of the train 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".
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.