When Labour’s Keir Starmer was elected as prime minister last month, he planned to tackle some of the long-term issues bedeviling Britain. His first priorities included reviving a stagnant economy and repairing the struggling National Health Service.

Now, he faces a more immediate and unexpected problem, one that is proving to be his government’s first major domestic test. England and Northern Ireland have been rocked by a week of anti-immigrant riots that have gripped cities and towns and escalated over the weekend. The riots came in the wake of a knife attack on a children’s dance class in the town of Southport, near Liverpool, that left three young girls dead, and the subsequent violence was driven by disinformation about the identity of the attacker.

He was not, as some claimed, an asylum seeker, but that made little difference as the misinformation ricocheted online, from private messaging channels to social media platforms like X. He was born and raised in Britain and, according to the BBC, his parents are from Rwanda.

Suddenly, Mr. Starmer and his government found themselves fighting a two-pronged battle, online and in the streets where rioters rampaged in more than 15 towns and cities, injuring dozens of police officers, looting businesses, targeting mosques and setting fires outside a hotel that housed asylum seekers.

So far, Mr. Starmer, a former chief prosecutor, has taken a law-and-order approach, vowing to crack down on perpetrators and bring charges swiftly, as well as putting more police on the streets and providing additional security personnel to Muslim communities.

And, so far, he has mostly avoided attacks from his political opponents, who have moved to unite in their opposition to the wanton violence. Still, Mr. Starmer faces challenges in dealing with the disorder that has badly shaken communities, partly, analysts say, because he inherited an overwhelmed criminal justice system.