Prime Minister Keir Starmer condemned the far right for the riots and backed police to crack down.
Far-right protesters have clashed with police at several demonstrations as unrest linked to misinformation about the murder of three girls in a stabbing attack spreads across the UK.
Riots involving hundreds of far-right anti-immigration protesters have broken out in several cities and towns in recent days after false information spread rapidly on social media that the suspect in Monday's knife attack at a children's dance class in Southport was a Muslim immigrant.
Police said the suspect, 17-year-old Axel Rudakubana, was born in Cardiff, Wales, but protests by anti-immigrant and anti-Muslim demonstrators have continued, leading to violence and rioting, including in the northeastern city of Sunderland on Friday night.
Protesters threw chairs, flares and bricks at officers in the north-west English city of Liverpool, while clashes broke out between police and protesters in nearby Manchester.
Merseyside Police said “a number of officers were injured while dealing with serious disorder” in Liverpool city centre.
In Belfast, Northern Ireland, fireworks were set off amid tense exchanges between an anti-Muslim group and an anti-racism protester.
Some businesses in the city reported material damage.
“I have no reason why we were attacked,” said Rahmi Akyol, standing outside his Belfast cafe, whose glass doors were smashed after being attacked by dozens of people throwing bottles and chairs.
“I have been living here for 35 years. My children and my wife are from here. I don’t know what to say, it’s terrible,” he said.
In Leeds, around 150 people carrying English flags chanted “You are not English anymore”, while counter-protesters shouted “Get off our streets, you Nazi scum”. In the central city of Nottingham, there were also clashes between groups of opposition protesters.
Authorities in the eastern city of Hull said four people were arrested and three officers injured as they dealt with protests involving bottle throwing.
During protests in London, police arrested several people, including one for giving a Nazi salute to a counter-protester.
Mosques across the country have been advised to step up security, while police have deployed additional officers.
Prime Minister Keir Starmer, facing his first major test since being elected a month ago, has condemned the “far right” for the violence and backed police to crack down.
Starmer held talks with senior ministers on Saturday and said there was “no excuse for violence” at the protests.
The last time widespread violence erupted in the UK was in 2011, when thousands of people took to the streets over five nights after police shot dead a black man in London.
On Friday night, hundreds of anti-immigration protesters in Sunderland threw stones at riot police near a mosque, before overturning vehicles, setting a car on fire and starting a fire near a police station.
Four injured police officers were taken to hospital and 12 people were arrested, Sunderland area police chief superintendent Mark Hall told reporters on Saturday.
“This was not a protest. This was unforgivable violence and disorder,” Hall said.
