UK police monitored several far-right protests and counter-demonstrations on Saturday after a third night of rioting linked to misinformation about a mass stabbing that killed three young girls sparked fears of more unrest.
The violence, which has seen scores of arrests across England and put Britain's Muslim community on edge, presents the biggest challenge yet of Labour Prime Minister Keir Starmer's month-old premiership.
It has also put hard-right agitators linked to football hooliganism in the spotlight at a time when anti-immigration elements are enjoying some electoral success in British politics.
By early afternoon, demonstrations were underway in several UK cities, including Manchester, Leeds, Nottingham, Portsmouth and London, as well as Belfast in the province of Northern Ireland.
There were no immediate reports of violence.
Police are on high alert after 10 people were arrested and four officers required hospital treatment following a riot in the northeastern English city of Sunderland late on Friday.
Footage 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 and a building next to a police station.
Other images shared on social media showed balaclava-clad youths throwing bricks and other missiles as fireworks and flares were let off, while shops were also looted.
"This was not a protest, this was unforgivable violence and disorder," Northumbria Police Chief Superintendent Mark Hall told reporters.
The unrest followed two nights of disturbances in several English towns and cities in the wake of Monday's frenzied knife attack in Southport, near Liverpool on England's northwest coast.
They were fuelled by false rumours on social media about the background of British-born 17-year-old suspect Axel Rudakubana, charged with several counts of murder and attempted murder over the attack at a Taylor Swift-themed dance party.
After violence in Southport late on Tuesday, unrest rocked the northern cities of Hartlepool and Manchester as well as London 24 hours later, where 111 people were arrested outside Starmer's Downing Street residence.
- 'Thugs' -
In Southport, the mob threw bricks at a mosque, prompting hundreds of Muslim places of worship across the country to step up security amid fears of more anti-Islamic demonstrations.
Police blamed supporters of the disbanded English Defence League, an anti-Islam organisation founded 15 years ago whose supporters have been linked to football hooliganism.
In Sunderland on Friday, rioters attacked police officers, set a police station and two cars on fire, and again targeted a mosque.
Anti-racism campaign group Hope Not Hate has identified more than 30 events planned for Saturday and Sunday.
Far-right social media channels have advertised "enough is enough" anti-immigrant rallies, while anti-fascism groups have vowed to stage counter-protests.
In Belfast, a small group gathered in the city centre and chanted "Islam out".
In Hull, one group of demonstrators chanted "You're not British any more," while counter-protesters shouted "Say it loud, say it clear, refugees are welcome here," the BBC reported.
In London, demonstrators attending a regular pro-Palestinian march appeared undeterred by an expected separate anti-immigration protest.
"My parents told me not to come today but I am from here. The UK is my home," 24-year-old student Meraaj Harun told AFP.
Counter protests were also expected in the central city of Nottingham while South Yorkshire police said they knew of a planned protest in the town of Rotherham.
Prime Minister Keir Starmer has accused "thugs" of "hijacking" the nation's grief to "sow hatred" and pledged that anyone carrying out violent acts would "face the full force of the law".
He has announced new measures that will allow the sharing of intelligence, wider deployment of facial-recognition technology and criminal behaviour orders to restrict troublemakers from travelling.
Labour politicians have accused Reform UK party leader Nigel Farage of stoking the trouble.
At last month's election, his anti-immigrant Reform UK party captured 14 percent of the vote -- one of the largest vote shares for a far-right British party.
By Peter Hutchison And Marie Heuclin