A so-called Brexit betrayal march led by controversial activist Tommy Robinson was “vastly” outnumbered by counter-demonstrators, opposition organisers said.
The English Defence League (EDL) founder and Ukip members marched with supporters through the streets of London on Sunday before a rally beside Parliament Square.
Amid fears of violence, Scotland Yard placed restrictions on the march as well as on a counter-demonstration organised by Labour supporters and anti-fascists.
Among those marching against the Brexit betrayal group were Labour grassroots group Momentum and Unite Against Fascism.
A Momentum spokeswoman said about 15,000 turned up to oppose Mr Robinson’s march, claiming it “vastly” outnumbered them nearly five to one.
A Ukip spokesman said “quite a few thousand” had turned up to its rally.
Police did not provide estimates on crowd sizes.
Momentum national co-ordinator Laura Parker said: “Today is a huge blow for Tommy Robinson and his vile, hate-fuelled politics.
“Even with the Ukip machine in tow he only managed to bring a few thousand supporters out on the streets while we mobilised nearly 15,000 to march against his racism and bigotry.”
Mr Robinson’s supporters gathered to call for Brexit, with one brandishing a noose he said was for Prime Minister Theresa May.
Shadow chancellor John McDonnell had urged Labour supporters to march against the “poison” of Mr Robinson, whose real name is Stephen Yaxley-Lennon.
“This march isn’t about Brexit, it’s about far-right extremists dressing up in suits and pretending to be respectable,” Mr McDonnell said.
Along the route, the counter-protest included songs and dancing, arriving at their final stop on Whitehall to the sound of Michael Jackson’s hit Black Or White.
As the demonstrators made their way from Portland Place, they held placards saying: “Oppose Tommy Robinson. Don’t let the racists divide us” and chanted “There are many many more of us than you”.
Marchers with their faces covered briefly made their way to the front of the march, and at a few points a handful of Robinson supporters were escorted away swiftly by police.
The protesters chanted the whole way, shouting “Nazi scum, off our streets”.
Gathering outside the Dorchester Hotel, Mr Robinson’s supporters marched along a specified route from Park Lane to Parliament Street.
Among Ukip supporters outside the luxury establishment was a man who gave his name as Laukan Creasey, from Stevenage, who was carrying a gallows with a noose hanging down.
Asked why he was brandishing it, Mr Creasey said: “That’s what the traitor May deserves. That’s what treasonous people get.
“It was a referendum not a never-endum. And they promised to implement whatever we decided and they haven’t, so two-and-a-half years down the line this is what you get.”
The Ukip group marched to outside the Houses of Parliament where they were to be greeted by festive music and speakers.
The crowd loudly booed when Ukip leader Gerard Batten brought up “Remainer” Mrs May and they cheered on his reference to a “treasonous political class”.
He added: “If Parliament does not take Britain out of the European Union it will be the biggest constitutional crisis since the English Civil War.
“In 1642 the king put himself in opposition to parliament. Parliament won and the king lost his head.
“If Parliament betrays Brexit they will be putting themselves in opposition of the people and if they win you will lose your liberty.”
Their opposition march, who had started outside the BBC building in Portland Place, were separated from the Brexiters by police barriers in Whitehall.
Police were on heightened alert after “serious violence” broke out at a Robinson rally in London in June, with five officers injured when bottles and barriers were hurled at them.
Scotland Yard said it also imposed the conditions based on the “current intelligence picture”.
Weyman Bennett, joint convener of Stand Up To Racism and one of the march organisers, said: “I believe that the majority of people in this country reject fascism and racism.
“There’s deep concern in Britain about the growth of the far right in this country, under the guise of Tommy Robinson and Ukip.”
He added: “We are excited about the amount of women organisers, Muslim groups and trade unions that have come out.
“We’ve had unprecedented unity. All of us have come together from whatever party or faction we represent and have agreed that we have to march together to defend our democratic rights.”