A “Brexit betrayal” march led by controversial activist Tommy Robinson was “vastly” outnumbered by counter-demonstrators, opposition organisers said.
The English Defence League (EDL) founder turned-Ukip adviser marched with supporters through the streets of London on Sunday before addressing 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.
A Ukip spokesman said “quite a few thousand” had turned up to its rally, which came ahead of MPs voting on a Brexit deal on Tuesday.
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.
Among those marching with Ukip were those carrying Generation Identity flags, which anti-fascist campaigners Hope Not Hate describe as a far-right and “strongly anti-Muslim” organisation.
Ukip supporters started their procession at the Dorchester Hotel and followed a specified route to the Houses of Parliament.
Outside the luxury establishment stood the man carrying a gallows with a noose hanging down. He gave his name as Laukan Creasey, from Stevenage, Hertfordshire.
Asked why he was brandishing it, he 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.”
Labour MPs were quick to criticise the noose, with Karen Buck saying it was “very, very wrong” and Alex Sobel adding: “This is what actual fascism looks like.”
Hope Not Hate said the march had been “absolutely riven with ugly messages and symbolism”.
At the march’s conclusion, 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 also conjured up a English Civil War metaphor to describe a potential failure to execute Brexit by politicians.
“In 1642 the king put himself in opposition to Parliament. Parliament won and the king lost his head,” he said, standing alongside Mr Robinson as well as Neil Hamilton, a former Tory politician who leads Ukip Wales.
He then hinted he could run for Parliament, after referencing “disgraceful lies” said about him by MPs including Commons Speaker John Bercow, who called him a “loathsome, obnoxious, repellent individual” in the Commons.
Mr Robinson told supporters it was then for the first time he thought: “One day, I’ll be sitting in there amongst you.”
His rally was separated from counter-demonstrators on Whitehall by police barriers and a vast divide.
The opposition march started outside the BBC building in Portland Place and arrived at its conclusion dancing to Michael Jackson’s hit Black Or White and Cha Cha Slide.
Along the route, 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.”
“Nazi scum, off our streets,” many added.
Marchers with their faces covered briefly made their way to the front, and at a few points a handful of Robinson supporters were swiftly escorted away by police.
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”.
The allegations were a public order offence, assaulting a police officer and possessing an offensive weapon.
A Stand Up To Racism representative told the crowd some 15,000 had attended the counter-demonstration.
Co-organiser and the organisation’s joint convener Weyman Bennett told the Press Association: “I believe that the majority of people in this country reject fascism and racism.
“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.”