British Prime Minister Keir Starmer has issued a stern warning that violent protesters targeting the country's Muslim communities would face "full force of the law" as he sought to quell days of anti-immigration rioting.

The fatal stabbing of three young girls in the northwest English town of Southport last week has been seized on by anti-immigrant and anti-Muslim groups, with disinformation spread online and amplified by high-profile far-right figures to spark disorder in towns and cities.

"Whatever the apparent motivation, this is not protest, it is pure violence and we will not tolerate attacks on mosques or our Muslim communities," Starmer said Monday after an emergency meeting with police and prison chiefs.

"The full force of law will be visited on all those who are identified as having taken part." he added.

Police chiefs said they had arrested 378 people since the start of the unrest and warned of "lengthy prison terms" for those found guilty of violent disorder.

The violence erupted last Tuesday after social media posts said the suspected attacker in Southport was a radical Islamist who had just arrived in Britain and was known to intelligence services.

Police say the 17-year-old suspect was born in Britain and they are not treating it as a terrorist incident. The suspect's parents had moved to Britain from Rwanda.

Interior Minister Yvette Cooper said rioters had felt "emboldened to stir up racial hatred" and that the protests were not a proportionate response to concerns about near-record levels of immigration.

"Reasonable people do not pick up bricks and throw them at the police," she said. Protests, mostly involving a few hundred people, have continued across the country, with shops looted and mosques and Asian-owned businesses attacked.

Cars have been set on fire and some unverified videos on social media have shown ethnic minorities being beaten up.

Australia and Nigeria were among countries to issue warnings on Monday to citizens resident in or traveling to Britain.

On Monday evening, protests spread to Plymouth in southwest England. Several hundred anti-immigration protesters wearing English and British flags faced off against a greater number of counter-protesters, kept apart by police in riot gear.

Protesters threw bricks and fireworks and scuffled with police. Sky News said three police officers were injured.

In Rotherham, northern England, protesters on Sunday tried to break into a hotel that housed asylum seekers in what Starmer called an act of "far-right thuggery", following protests on Saturday in other English cities and in Belfast.

Starmer said a "standing army" of specialist police officers would tackle outbreaks of violence where needed. Northern Ireland's assembly will end its summer break a day early to discuss the violence.

Online disinformation

Police have blamed online disinformation, amplified by high-profile figures for driving the violence.

Stephen Yaxley-Lennon, known by the pseudonym Tommy Robinson and previously the leader of the defunct anti-Islam English Defence League, has been blamed by media for spreading misinformation to his 875,000 followers on X.

"They are lying to you all," Yaxley-Lennon said. "Attempting to turn the nation against me. I need you, you are my voice."

Elon Musk, the owner of X, also weighed in. Responding to a post on X that blamed mass migration and open borders for the disorder in Britain, he wrote: "Civil war is inevitable."

Starmer's spokesperson said there was "no justification" for Musk's comment. Musk later criticised Starmer for a post on X which identified mosques as needing particular protection.

In Whitechapel in London, lawyer M. A. Gani, 33, said the British Bangladeshi community was "living in fear."

"We've never seen this kind of far-right groups (being so) active and anti-immigrant," he said.

"I hope that the U.K. (government) will take initiative to calm down the situation."

Britain's technology minister, Peter Kyle, met representatives of social media platforms including X to remind them of their responsibility to stop the spread of racial hate and incitement to violence.

"There is a significant amount of content circulating that platforms need to be dealing with at pace," he said.