More than 50 police were injured in an outbreak of violence driven by false reports that the suspect in the stabbing attack on a children’s dance class was a migrant.

Riot police officers holding back protesters in Southport, England, on Tuesday.

Less than two hours after mourners gathered in Southport, England, on Tuesday evening to honor three children killed in a brutal stabbing attack, hundreds of rioters flooded the streets of the already traumatized town.

More than 50 police officers were injured in the ensuing violence, as demonstrators threw bricks at a mosque, attacked the police, set cars on fire and damaged a convenience store.

Although some details of the unrest remain opaque, one thing is clear, according to the police, lawmakers and experts in online extremism: Disinformation and far-right agitators fueled the violence.

Supporters of the English Defence League, an extremist anti-Islam organization, were part of a large group that attacked a mosque in Southport around 7.45 p.m., according to a statement from the Merseyside Police Service, which covers the region.

The targeting of the mosque, and the subsequent riot, came after false rumors circulated on social media on Monday, soon after news emerged that a man had stabbed multiple children and two adults at a Taylor Swift-themed dance class.

The rapid spread of misinformation about the attacker’s identity left the authorities fighting a two-pronged battle on Tuesday: one on the streets of Southport, where the police were pelted with bricks and other objects, and another online, where lawmakers, local officials and the police seemed powerless to halt viral falsehoods.

On Monday afternoon, the police said they had arrested a 17-year-old in the stabbings. On Wednesday, he was charged with three counts of murder and 10 counts of attempted murder, a police statement said. In line with British law regarding minors, the police did not identify the suspect, but said he lived in the nearby village of Banks.

On X, the social media platform owned by Elon Musk, users shared false information about the attacker. Some posted what they claimed was the attacker’s name, which the police said was incorrect, but that information continued to spread. Others spread falsehoods about the attacker’s immigration status, incorrectly claiming he was an asylum seeker or that he had come to England illegally. Some of the posts received millions of views, fanning the flames of far-right narratives that oppose immigration.

Tommy Robinson, an anti-Islam agitator who founded the English Defence League, and Andrew Tate, another extremist online personality, were among those who fueled speculation.

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Mr. Robinson shared a social media post in which a man asked: “Why has our government let this Syrian fella in” to stab “innocent children.”

Mr. Tate posted a video on X on Monday night in which he told viewers that an “undocumented migrant decided to go into a Taylor Swift dance class today and stab six little girls, so someone arrived in the U.K. on a boat, nobody knew who he was, nobody knows where he’s from.” The video has been viewed more than 14.9 million times.

As misinformation about the suspect spread, the police issued statements saying that he was born in Cardiff, Wales. But false claims continued to proliferate.

Since Mr. Musk acquired X in 2022, he has rolled back many of the platform’s content- moderation policies, reinstated the accounts of previously banned extremists, including Mr. Robinson, and laid off workers responsible for policing misinformation.

Instead, Mr. Musk has favored an approach that allows X users to fact-check one another’s posts. The program, called Community Notes, began in 2021 but expanded rapidly under Mr. Musk. Several of the false posts that were viewed widely on X, including Mr. Tate’s, received Community Notes that pushed back on the misinformation, even as the posts remained visible.

As the riot took place on Tuesday night, Alex Goss, the assistant chief constable of the Merseyside police, said in a statement, “There has been much speculation and hypothesis around the status of a 17-year-old male who is currently in police custody and some individuals are using this to bring violence and disorder to our streets.”

He added, “We have already said that the person arrested was born in the U.K. and speculation helps nobody at this time.”

Many of those involved in the unrest, he noted, “do not live in the Merseyside area or care about the people of Merseyside.”

The outbreak of violence was deeply distressing for a community still grappling with raw emotions after Monday’s attack. Thousands had gathered for a vigil to honor the victims, crowding a central square and grasping bouquets of flowers and teddy bears.

In an interview on Wednesday morning with BBC radio, the lawmaker who represents the area, Patrick Hurley, said the rioters were “utterly disrespecting the families of the dead and injured children” and called them “beered-up thugs” who were not from Southport. He added, “Even if this lad, the 17-year-old, turns out to be Muslim, under no circumstances does that justify any attack on a mosque.”

The police have said that they are still investigating the motive for the attack, but that it was “not being treated as terrorist-related.”

Prime Minister Keir Starmer also condemned the rioters and vowed to hold those who took part accountable. “Those who have hijacked the vigil for the victims with violence and thuggery have insulted the community as it grieves,” Mr. Starmer wrote in a statement. “They will feel the full force of the law.”

Mr. Robinson denied in a social media post on Wednesday that the English Defense League still existed. But experts say that while its membership declined in the 2010s, its supporters have continued to mobilize around certain events.

The Merseyside Police said their officers had suffered serious injuries during the unrest, including fractures, lacerations, a suspected broken nose and a concussion. Others had head injuries and facial injuries, and one was knocked unconscious, the force said. The North West Ambulance Service, which covers the region, said that 39 patients, all of whom were police officers, had been treated for injuries, with 27 taken to the hospital.

The same police and ambulance services had been the first responders the community relied on one day earlier, Serena Kennedy, the chief constable for the area, pointed out. Then, they, too, were targeted. She told reporters on Wednesday that the police were prepared for more possible violence in the coming days.

But the unrest was not isolated to Southport. On Wednesday evening, a far-right demonstration outside government buildings in London turned violent, with protesters clashing with the police. The group chanted, “We want our country back” and “England ’til I die.”

The Metropolitan Police of London said more than 100 people had been arrested on offenses including violent disorder and assault on an emergency worker. Some officers also suffered minor injuries, the police said.

Nick Lowles, the chief executive of Hope Not Hate, a British anti-extremism watchdog group, said that the horrific nature of the attack had evoked strong emotions, which had then been inflamed by misinformation.

“It’s far more than just the traditional far right; some of these narratives are in the mainstream now,” he said. “And each time a more mainstream person says it, it gives it more legitimacy.”

Mr. Lowles pointed to the normalization of extreme anti-immigration attitudes in recent years. “We’ve have had the rise of Nigel Farage and Reform U.K., whose rhetoric is not as violent but no less extreme,” he said, noting that Mr. Farage, a newly elected lawmaker and leader of the insurgent hard-right party, had put out a video questioning official information about the attacker.

The violence occurred a short walk from where the initial stabbings took place. As the evening light turned to darkness on Tuesday, smoke from the riot drifted over the memorial that had sprung up, the acrid black fumes visible on the horizon over the piles of flowers.

Mr. Goss, of the Merseyside Police, said that a number of off-duty police officers had been called in to support their colleagues, and he applauded their dedication while criticizing others who had taken part in the disorder.

“This is no way to treat a community,” he said, “least of all a community that is still reeling from the events of Monday.”