A total of 22 people - including children - were injured in the explosion, with majority suffering from "flash burns and are being treated in hospital, mostly for burn injuries".
One witness said there were "lots" of people injured in an explosion and the subsequent rush to flee the train and station. As a manhunt takes place across the capital, authorities have tightened security on all transport hubs, The New York Times reports.
Passengers suffering burns and covered in blood were being transported out of Parsons Green station in Southwest London following the explosion. London ambulance service said they had sent multiple crews to the Parsons Green station, and 18 people were hospitalised, though none had life-threatening injuries.
Pictures shared on social media showed embers burning from a white bucket which had been wrapped in a plastic shopping bag. While some of the passengers were burnt by the explosion, others were injured in a stampede to escape.
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Transit officials increased security Friday at Bay Area BART stations in the wake of a terrorist bombing in London that left 22 people injured. Many have been told to stay in lockdown as police evacuated the area and placed a wide zone under a police cordon.
London has been struck repeatedly by extremist attacks this year, and the risk of new attack is considered high. Scotland Yard said no arrests have been made yet in connection with the attack.
London police are being supported by Britain's MI5 intelligence service, he said.