Police in London said they arrested more than 100 people on Saturday after far-right protesters holding a counter-demonstration against anti-racism activists clashed with officers.
Thousands of people defied the coronavirus restrictions to assemble in and around Parliament Square, in the centre of the capital.
That required "major" policing operation the Metropolitan Police Service said, adding that they had encountered "pockets of violence directed towards our officers".
Television footage showed some agitators throwing punches, bottles and smoke bombs at officers as well as scuffling with rival protesters.
The Met they had arrested more than 100 peopled by 2000 GMT for violent disorder, assault on officers and possession of an offensive weapon. Six officers suffered minor injuries.
Prime Minister Boris Johnson said "racist thuggery has no place on our streets" and that "anybody attacking the police would be met with full force of the law".
As footage of the disorder was shared widely on social media, he wrote on Twitter: "Racism has no part in the UK and we must work together to make that a reality."
'Guard our monuments'
A protest by the Black Lives Matter group planned for Saturday had been switched to Friday to avoid clashes with the far-right counter-demonstration.
Anti-racism group Hope Not Hate had warned before Saturday that hooligan gangs attached to some English football clubs also planning to attend Saturday's counter-protest.
Paul Golding, leader of fringe far-right political group Britain First, which has seen its members jailed for hate crimes and been banned from Facebook, was among the first to arrive at Parliament Square.
Golding, who was last month found guilty of an offence under the Terrorism Act, told the domestic Press Association news agency they had turned out to "guard our monuments".
Earlier, a march by several hundred Black Lives Matter activists through the capital went ahead, ending in Trafalgar Square near where the counter protesters gathered and amid a heavy police presence.
Police commander, Bas Javid said a number of demonstrators had failed to disperse by 1600 GMT.
"There have been pockets of violence directed towards our officers. This is completely unacceptable and I condemn those involved," he added.
Police have also launched an investigation after a photo posted on social media appeared to show a man urinating on a memorial to Keith Palmer, the police officer killed in the 2017 terror attack outside parliament.
The Met would "gather all the evidence available to us and take appropriate action," said Javid.
London's mayor Sadiq Khan praised the force for doing a "fantastic job to control the situation.
"Millions of Londoners will have been disgusted by the shameful scenes of violence, desecration and racism displayed by the right-wing extremists who gathered in our city today," he added.
Britain has seen a wave of protests prompted by last month's death during a US police arrest of George Floyd, an unarmed African-American, which triggered outrage around the world.
Most of them have been peaceful, but demonstrations in London last weekend latterly turned violent, while crowds in Bristol, southwest England, toppled a statue to a 17th-century slave trader Edward Colston and threw it into the harbour.
Several central London memorials were boarded up as a precaution ahead of Saturday's demonstrations. They included one of World War II leader Winston Churchill -- which last weekend was defaced with the word "racist" -- and the Cenotaph war shrine.
Former Conservative MP Nicholas Soames, Churchill's grandson, denounced the "very small, extremely explosive group of people" responsible for last weekend's vandalism.
But he told the Daily Telegraph: "The idea that the hard right should stand guard over Churchill is absolutely repulsive.
"It feels like a society that has lost its compass."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)