Several police officers have been injured and a police van was damaged as rival protest groups faced off in Plymouth.
The crowds were held apart by lines of riot police, with violence breaking out as protesters threw rocks and fireworks.
Trouble centred on a crowd of anti-immigration protesters who attempted for several hours to confront a Stand Up To Racism counter-rally.
Ever since a fatal knife attack in Southport last week, groups with links to the far right have taken to the streets in towns and cities across England.
They have repeatedly and violently clashed with police separating them from counter-protesters, throwing missiles, damaging buildings and looting shops.
UK riots latest: Riot police standing between rival protest groups in Plymouth
Hundreds of people have been arrested so far, in unrest that has targeted mosques in UK cities.
Several officers were injured in the port city after far-right rioters broke through police lines.
The disorder was described by one senior police officer as “sustained violence”.
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Speaking to Sky News’ West of England correspondent Dan Whitehead, the bronze commander with Devon and Cornwall Police said: “This is not a protest anymore. In my opinion, this is violence. This is sustained violence.”
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Inspector Ryan Northmore added: “Unfortunately, large masonry was thrown at us. Whether it was aimed at us, or others, or opposing groups, we don’t know yet.”
Fireworks were also thrown from the far-right crowd into the counter-protest group, along with bottles and bricks.
Throughout the night, members of the far-right group moved up and down the street trying to find holes in police lines.
Meanwhile, the Stand Up To Racism protesters largely stayed in place.
Devon and Cornwall Police said, during the evening, that “arrests are ongoing” after they were met with “a level of violence”.
Later on in the evening, police tried to push back the far-right protesters as they worked on several fronts to contain the groups.
Six people were arrested, with police saying: “People who were intent on committing public order or criminal offences, have and will be dealt with robustly.
“This is the style of policing that we have delivered and what our communities expect.”
Plymouth’s MP said that the violence does not represent the area.
Luke Pollard, who is also armed forces minister, said in a video on X that “there is absolutely no place for violence on our streets”.
He added: “Plymouth is a brilliant place to live, work and play. The diversity of our city makes us stronger – doesn’t make us weaker.
“That’s not what Plymouth is about – that’s not who we are. Thank you to the police for keeping us safe, to services from the city council to local businesses who have all taken steps to reassure people, to keep people safe.”
Sky News crew confronted in Birmingham
Meanwhile, in Birmingham, word spread among the Muslim community that a far-right protest had been planned in the city.
One community activist told Sky News that local men planned to stand up to them.
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As Sky News’ Becky Johnson tried to report on the incident, the crew was told to leave by a group of men.
Despite reassurances from older men, the situation became more hostile and as they left a man in a balaclava stabbed at the tyre of their broadcast van.
First rioters in court
Following hundreds of recent arrests, the first group of people involved in the riots appeared in court.
The youngest of those to appear in court was a 14-year-old boy who cannot be named for legal reasons.
He said he understood how “foolish and silly” he had been after pleading guilty to violent disorder over the rioting in Liverpool city centre, a court heard.
Liverpool Magistrates’ Court heard the teenager, from Liverpool, was part of a group of eight to 12 males who were lighting fireworks and setting them off in the direction of members of the public and police officers near a branch of B&M in Clayton Square that had already been looted on Saturday night.
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Southport knife attack
The violence and unrest follows the knife attack on a Taylor Swift themed dance class that left three young girls dead.
They were named as six-year-old Bebe King, seven-year-old Elsie Dot Stancombe, and nine-year-old Alice Dasilva Aguiar.
Axel Rudakubana, 17, has been arrested on suspicion of murder and attempted murder.
Police have said he is from the nearby village of Banks in Lancashire, but was born in Cardiff, Wales.
Following the incident, misinformation spread online claiming he had come to the UK on a small boat.
Anti-immigration groups used this as fuel for protests, which quickly turned violent and spiralled into the ongoing episodes of disorder.
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As violence broke out in Plymouth on Monday evening, in Southport, a tribute was held for victims one week on.