LONDON — The Kurdish-Iranian teenager was waiting with friends at a bus stop in south London when a gang of men and women in their 20s cornered him and asked him in aggressive tones where he was from.

When he replied that he was an asylum seeker, the police and news reports said, they chased him through the streets and finally caught him, throwing him to the ground and repeatedly punching and kicking him in the head, even as he screamed for help.

At the sound of police sirens, the attackers fled, leaving the teenager unconscious, with a fractured skull and a blood clot in his brain. As many as 20 people may have participated in the Friday night attack, which the police are treating as a suspected hate crime.

On Monday, the frenzied assault on the 17-year-old, described by the police as “brutal,” was reverberating across Britain amid growing concerns that the country’s decision to leave the European Union, or “Brexit,” had spawned an anti-immigrant backlash.