LONDON (AP) — A homemade bomb exploded on a packed London subway train during rush hour on Friday, leaving 22 people injured but no one seriously hurt. Police said the explosion was a terrorist attack, the fifth in Britain this year.

Commuters reported a noise and a flash aboard the District Line train at Parsons Green station in the southwest of the city, followed by chaos as hundreds of people rushed to get away from danger.

Mark Rowley, head of counterterrorism for the Metropolitan Police, said “we now assess this was a detonation of an improvised explosive device.”

He said 22 people had been injured, most with “flash burns.”

Photos taken inside the train show a white plastic bucket inside a foil-lined shopping bag. Flames and what appear to be wires emerge from the top.

“There was out of the corner of my eye a massive flash of flames that went up the side of the train,” eyewitness Chris Wildish told Sky News, then “an acrid chemical smell.”

He said many of those on board were schoolchildren, who were knocked around as the crowd surged away from the fireball.