LONDON — The authorities in Britain said on Monday that they were treating an early morning attack near a mosque in London as an act of terrorism against Muslims, amid fears of retaliation for several recent assaults in the country attributed to Islamist extremists.

Shortly after midnight, a van rammed into a group of pedestrians near the Finsbury Park Mosque, in North London, and the imam of a nearby community center was credited with preventing an angry mob from attacking the driver after worshipers subdued him.

One person died at the scene and at least 10 were wounded, but the authorities said it was not immediately clear if the death was caused by the attack. The assailant was identified by the authorities as Darren Osborne, a 47-year-old man from Cardiff, Wales.

Little was immediately known about Mr. Osborne or his motives. Jo Stevens, the member of Parliament for the Cardiff Central constituency, issued an appeal to the public for any information about him but also urged people to refrain from online speculation.