LONDON — Alex Ojeda-Sierra, 13, was on the train to school with a friend when they heard screaming and saw passengers running past.

Unknown to the boys, a bomb had exploded in another car.

“I dropped my bag and we started running,” Alex, who attends the London Oratory School, said from a wheelchair at Chelsea and Westminster Hospital, where he was treated for facial bruises and sprains when he tripped in the panicky crush of fleeing commuters.

“One man fell on me and I had my legs bent backwards and my right ankle got twisted, and I started screaming that I had no air,” he said.

The bomb, wrapped in a plastic grocery bag concealed in a bucket, exploded at 8:20 a.m. Friday at the height of the morning rush. The explosion and panic left 29 people injured, but none were killed.

It was the fifth terrorist attack in Britain this year and the first to hit London at its most vulnerable point — mass transit — since the 2005 bombings that killed 52.

The Islamic State asserted responsibility for the bucket bomb hours later in a message on its Amaq news site that said a “detachment” of its disciples had carried out the attack — language that suggested more than one assailant.