Rioting flared late on Friday in Sunderland, northeast England, where police said officers were subjected to "serious violence," the latest unrest following a mass stabbing earlier this week that killed three young girls.

Footage shared on social media and broadcast by the BBC showed a mob of several hundred rampaging in Sunderland's city centre, attacking police and setting fire to at least one car.

It came as police nationwide prepared for planned far-right rallies and other demonstrations this weekend, after two nights of unrest in several English towns and cities in the wake of Monday's knife attack.

London's Metropolitan Police said it had a "proportionate and risk-based" plan for rival pro-Palestinian and anti-immigration protests on Saturday. Late on Wednesday, its officers arrested 111 people as a rally in Westminster turned violent.

But Friday night's disturbances in Sunderland were an ominous sign for the coming days.

"Our officers are continuing to deal with ongoing disorder and have been subjected to serious violence," Northumbria Police said of the situation there.

"The scenes that we are seeing are completely unacceptable," it added.

Regional mayor Kim McGuinness said she was "appalled" at the fresh eruption of violence, this time in the northeast, which followed a calm night on Thursday across the country.

"If your response to tragedy is to use it to commit violence, to abuse others, attack the police and damage property you stand for nothing except thuggery," she said on X, echoing comments by Prime Minister Keir Starmer on Thursday.

Earlier, Merseyside Police said extra officers had been deployed in and around Liverpool city centre "to help prevent disorder or criminality" there.

That followed social media reports of planned far-right gatherings and counter-protests in the northwest English city, which lies close to the seaside town of Southport where Monday's knife attack occurred.

Far-right elements have been blamed for the disturbances in the aftermath of the stabbing spree targeting children at a Taylor Swift-themed dance party.

Officials have said online misinformation about the British-born 17-year-old suspect Axel Rudakubana has helped fuel the unrest.

He faces murder and attempted murder charges over the stabbing rampage that also left five children and two adults critically wounded and which has shocked Britain.

Police have said the case is not being treated as terror-related but have not revealed a motive.

Anger over the tragedy has fused with growing anti-immigration sentiment on the right of English politics, pushed by openly Islamophobic far-right figures.

After violence in Southport late on Tuesday, unrest rocked the northern cities of Hartlepool and Manchester as well as London 24 hours later.

In Southport, the mob targeted a mosque, prompting hundreds of Muslim places of worship across the country to step up security amid fears of more anti-Islamic demonstrators.

Faith leaders says mosques have been hiring paid security staff and checking their CCTV cameras are working.

"The Muslim community is deeply anxious right now," said Zara Mohammed, secretary-general of the Muslim Council of Britain (MCB).

Starmer has accused "thugs" of "hijacking" the nation's grief to "sow hatred" and pledged that anyone carrying out violent acts will "face the full force of the law."

In power for less than a month, the prime minister met on Friday with local leaders in Southport to discuss "a package of support for the local community," his Downing Street office said.

Starmer's visit came the day after he announced a new "national capability" to tackle disorder in the wake of the recent disturbances.

The new measures will allow the sharing of intelligence, wider deployment of facial-recognition technology and criminal behaviour orders to restrict troublemakers from travelling.

Police nationwide were mobilising for trouble this weekend, after far-right social media channels advertised "enough is enough" anti-immigrant rallies and anti-facism groups vowed to stage counter-protests.

"Enough is enough" has emerged as a rallying online slogan for the far-right and others following Monday's attack.

South Yorkshire police said on Friday it knew of a planned protest in the town of Rotherham and that people might see "an increased police presence."

Officers in south Wales said a policing operation would be in place for events being planned for Saturday in Cardiff.

Police in Northern Ireland noted social media posts calling for roadblocks "using women and children," as well as a march to an Islamic centre in Belfast on Saturday. (AFP)