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.
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.

Swedish police on Friday said they seized a suspected bomb-laden car during a traffic check on the Malmo-Lund highway in Lomma.
A statement revealed that suspicious objects were found and the driver, a 45-year-old man, was arrested.
The highway in southern Sweden was temporarily closed as bomb disposal units investigated the vehicle.
Swedens security forces have been on increased alert in recent months.
On April 7, five people were killed and 15 others injured in an attack in the capital Stockholm as a stolen truck was rammed into a department store.
Reporting by Atila Altuntas:Writing by Ilker Girit

"I can't say very much about the case, for various reasons. We have to wait to hear what this person says in interrogations. A lot of things are not yet clear," police spokesperson Kim Hild told the TT newswire.

Two women were injured after a hammer-wielding man assaulted them while screaming "Allahu Akbar" in Chalon-sur-Sane, France, according to reports.
The attacker, wearing all black, is still on the run, French media reports.
The two assaults happened near Place de Beaune, 15 minutes apart.
One of the two women is believed to have been hit in the back of the head.
Chalon-sur-Sane is a town in the Sane-et-Loire department in the region of Bourgogne-Franche-Comt, in eastern France.

knife-wielding man attacked a soldier on patrol as part of an anti-terror operation in Paris but did not cause any injuries, police said.
The attacker allegedly lunged at the soldier at the central Chatelet metro station, one of the busiest transport hubs in the French capital, around 6.30am local time, making reference to Allah, a police source said.
The soldier quickly brought the man under control.
Soldiers and police officers have in recent years frequently been the target of attackers in France who often claim allegiance to the Islamic State of Iraq and the Levant (Isil) terror group.
French defence minister Florence Parly said the fact that the attacker was swiftly brought under control Friday was "proof of the professionalism and efficiency of the Sentinelle soldiers in their mission to protect".