Thousands of people have taken to the streets in London to march in a UKIP-organised rally and a counter-protest march.
A UKIP spokesman said "quite a few thousand" supporters turned up to their "Brexit Betrayal" march alongside controversial activist Tommy Robinson.
Marching in opposition were anti-fascist groups and Labour-backed campaigners.
A spokeswoman said 15,000 turned up to oppose Mr Robinson's rally.
Laura Parker of Labour grassroots group Momentum claimed their counter-demonstration "vastly" outnumbered UKIP's "nearly five to one".
She said: "Even with the UKIP machine in tow, he [Robinson] 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."
Police have not provided an estimate of crowd sizes.
At the scene
By Richard Galpin
Despite fears there could be clashes between the pro and anti-Brexiteers today, the marches which streamed through the streets of central London passed off peacefully.
Thousands of people took part on both sides of the divide, making their feelings known ahead of the key vote in parliament due to take place on Tuesday.
Those marching under the UKIP banner called for Theresa May's Brexit deal to be dumped.
One demonstrator carried an effigy of the prime minister describing her as a traitor, while another held a model scaffold and hangman's noose.
Later in a rally close to parliament and Downing Street, the UKIP leader Gerrard Batten said it had been an achievement to get the far-right activist Tommy Robinson involved in the demonstration.
Mr Robinson who denies claims he is a racist and fascist, joined the front of the march saying it was "a beautiful day."
Scotland Yard imposed restrictions on both marches and urged people to protest peacefully.
Mr Robinson and his supporters gathered outside the Houses of Parliament where they were addressed by UKIP leader Gerard Batten.
The former English Defence League leader was recently appointed as an adviser to Mr Batten, prompting a number of people to resign from the party, including former leader Nigel Farage,
Mr Batten told the crowd: "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."
Weyman Bennett, of Stand Up To Racism and one of the counter-protest 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.
UKIP had earlier predicted its "Brexit Betrayal" march would be "the largest pro-Brexit event of the year", with party leader Gerard Batten telling supporters it was the "only pro-Brexit rally to be held before the vote next week".
Labour's shadow chancellor, John McDonnell, had urged people to join the rally against Mr Robinson, whose real name is Stephen Yaxley-Lennon.
Scotland Yard said it had imposed conditions - which include the marches sticking to planned routes and limits on the time they could take place.
Mr Robinson and his supporters met outside the Dorchester hotel on Park Lane before marching along a pre-determined route to Westminster.
Police barriers separated them from the counter-protesters, who had gathered outside the BBC's Portland Place headquarters prior to marching to Whitehall.