Police officers have been assaulted as they clashed with protesters at a "freedom of speech" festival led by far-right activist Tommy Robinson, Scotland Yard has said.
More than 100,000 people were estimated by police to have gathered in London for the "Unite the Kingdom" event which faced counter protests by anti-racism campaigners.
The Metropolitan Police said it estimated around 110,000 were in Whitehall for the Robinson event and 5,000 for the counter protest.
In a statement, police said the turnout was too big to fit into the Whitehall area, leading to protesters leaving the route and seeking access to Whitehall from various directions.
The force, which was attempting to keep the two protest groups apart, said: "Officers are having to intervene in multiple locations to stop Unite the Kingdom protesters trying to access sterile areas, breach police cordons or get to opposing groups.
"A number of officers have been assaulted."
The Metropolitan Police said nine people have been arrested during the demonstration as officers faced "unacceptable violence".
"When officers moved in to stop them they faced unacceptable violence. They were assaulted with kicks and punches. Bottles, flares and other projectiles were thrown."
Earlier it said its officers had also been "attacked with projectiles and have had to use force to avoid their cordon being breached".
Watch: Anti-immigration protesters rally in central London
The two groups were divided by a line of police officers on Whitehall, one side waving placards that said "refugees welcome. Stop the far right" and the other flying Union and St George's flags.
The "Unite" protest featured speeches on a stage in Whitehall from Robinson and other activists including former actor Lawrence Fox, former Apprentice candidate Katie Hopkins along with musical performers and a group of bare-chested members of the Destiny Church in New Zealand performing a haka.
A sea of flags could be seen in the crowd including the St George’s Cross, the Union flag, the Scottish saltire and Welsh dragon with others carrying wooden crosses with "Christ" written on and singing Christian songs.
They chanted slogans critical of British Prime Minister Keir Starmer and carried placards including some saying "send them home". Some attendees brought children.
Elon Musk, speaking by videolink to the protest, said: "I think there's something beautiful about being British and what I see happening here is a destruction of Britain, initially a slow erosion but rapidly increasing erosion of Britain with massive uncontrolled migration."
'We believe in Tommy' - supporter
Robinson has billed the Unite the Kingdom march as a celebration of free speech. It is also expected to mourn Charlie Kirk, the American conservative activist shot dead on Wednesday.
"Hundreds of thousands already pack the streets of central London as we Unite as one for our freedoms," Robinson said on X.
Robinson, whose real name is Stephen Yaxley-Lennon, describes himself as a journalist exposing state wrongdoing and counts US billionaire Elon Musk among his supporters.
Britain's biggest anti-immigrant political party, Reform UK, which has topped opinion polls in recent months, has kept its distance from Robinson, who has several criminal convictions.
"We want our country back, we want our free speech back on track," said Sandra Mitchell, a supporter attending the rally.
"They need to stop illegal migration into this country," she said. "We believe in Tommy."
London's Metropolitan Police has said it has more than 1,600 officers deployed across London today, including 500 brought in from other forces.
In addition to policing the two demonstrations, the force is stretched by high-profile soccer matches and concerts.
Watch: Counter protest in London as thousands march against immigration
"We will approach them as we do any other protests, policing without fear or favour, ensuring people can exercise their lawful rights but being robust in dealing with incidents or offences should they occur," said Commander Clair Haynes, who is leading the policing operation.
Ms Haynes said police were aware of a record of "anti-Muslim rhetoric and incidents of offensive chanting by a minority" at previous protests, but said London's communities should not feel like they have to stay at home.
Last Saturday, nearly 900 people were arrested at a London demonstration against a ban on protest group Palestine Action.
Immigration has become the dominant political issue in Britain, eclipsing concerns over a faltering economy, as the country faces a record number of asylum claims. More than 28,000 migrants have arrived in small boats across the Channel so far this year.
Red and white English flags have proliferated along streets and been painted on roads. Supporters call it a spontaneous campaign of national pride, but anti-racism campaigners see a message of hostility to foreigners.
Additional reporting Reuters
