Tens of thousands of demonstrators filled central London on Saturday, waving English and British flags at a rally led by anti-immigrant and anti-Islam campaigner Tommy Robinson.
Police have said they will have a huge presence in the British capital.
A "Stand Up to Racism" counter protest also met nearby, following a highly charged summer in Britain that has seen protests over immigration and free speech.
By midday tens of thousands of protesters were packed into streets south of the River Thames, before heading towards Westminster, seat of the UK parliament.
Police said some 110,000 people attended the far-right rally.
Demonstrators carried the Union flag of Britain and the red and white St George's Cross of England, while others brought American and Israeli flags and wore the MAGA hats of U.S. President Donald Trump. They chanted slogans critical of Prime Minister Keir Starmer and carried placards including some saying "send them home". Some attendees brought children.
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 U.S. 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 will have more than 1,600 officers deployed across London on Saturday, 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.
"We will approach them as we do any other protests, policing without fear or favor, 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.
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.
