Video report by ITV News Reporter David Wood
One million people are estimated to have marched through London calling for the public to be given the final say on Brexit.
Demonstrators calling for a second referendum flooded into the capital from across the country for the Put It To The People march to Parliament Square on Saturday.
Political Correspondent David Wood on why he believes that Saturday's march will not persuade Theresa May to hold a second referendum
Around 700,000 took part in a similar rally in October, but People's Vote - the organisers behind the march - have estimated that more than one million people took part in Saturday's protest.
The huge size of the crowd saw people spill over into the capital's side streets and some underground trains were not stopping at Green Park station.
Watch as protesters make their way through central London
Marching bands, music, whistles, chants and cheers provided a noisy backdrop to the march, with demonstrators decked out in blue and yellow berets and flying large EU flags above the crowd as it makes its way slowly to Parliament Square.
Placards bearing messages urging the Government to "revoke Article 50" and for Brexit to be put to the people were carried by many protesters, while many others wore wearing fluorescent stickers reading "B******* to Brexit. It's not a done deal".
The day’s activities were kicked off by the unfurling of a large banner on Westminster Bridge that read “Love socialism, hate Brexit”.
The stunt was organised by a group calling itself the “Left Bloc” which is supported by Labour MPs, including Clive Lewis and Kate Osamor, Green Party MP Caroline Lucas, trade unions and grassroots campaigners.
Campaigners marched from Park Lane to Parliament Square, beginning at midday, followed by a rally in front of Parliament.
There, Labour’s deputy leader Tom Watson was one of many politicians and public figures to address the assembled masses.
Mr Watson told crowds he has reluctantly come to the view that the only way to resolve the current impasse is “for people themselves to sign it off”.
The West Bromwich East MP told the crowd: “The current impasse is not working for people who voted to leave or people who voted to stay.
"I really don’t think Parliament will be able to resolve this.
“That’s why I’ve come to the reluctant view that the only way to resolve this and have legitimacy in the eyes of the public is for the people themselves to sign it off.
“It can only bring closure if we’re all involved in making the decision.”
Mr Watson has previously said that if Prime Minister Theresa May agrees to a people's vote on the final deal, he will back her Brexit deal in a third meaningful vote in the Commons.
However, Mrs May has repeatedly ruled out a referendum.
Scottish First Minister Nicola Sturgeon also addressed the crowd, telling them that Mrs May had pitched "Parliament against the people".
"If that is your view, Prime Minister, let the people speak," she said.
Ms Sturgeon also accused Mrs May of being "in thrall to hardline Brexiteers", and called on Labour leader Jeremy Corbyn to back a People's vote.
Also speaking from the stage in Parliament Square, Mayor of London Sadiq Khan said he was "a proud European".
"No matter how you voted in the referendum, no matter what political party you support, we can all agree that Brexit has been a complete and utter mess," he said.
"With days to go we're in danger of falling of the cliff, which will have catastrophic consequences."
Mr Khan said it was "time to withdraw Article 50".
He added: "it's time to give us, the British people, a final say on Brexit."
Other keynote speakers include former Conservative cabinet minister Justine Greening and former attorney general Dominic Grieve, former Tory turned independent MP Anna Soubry, Lib Dem deputy leader Jo Swinson, Green Party MP Caroline Lucas and SNP Westminster leader Ian Blackford.
One of those to travel the furthest to the protest was Sorcha Kirker who undertook a 715-mile journey on ferries, trains and buses from Orkney in Scotland to take part.
The 27-year-old student is the vice president for higher education of the Highlands and Islands Students’ Association, and studies archaeology at Orkney College UHI.
She said she was joined by about 30 other students from the University of the Highlands and Islands.
She added: “Personally I think it’s really unfair because predominantly students and young people don’t want Brexit, and many were not old enough to vote in 2016, yet we are the ones whose futures it will affect for the longest.”
The march comes on the same day pro-Brexit campaigners continue their long hike from the North East to London, with former Ukip leader Nigel Farage re-joining the March to Leave as it set off from Linby, near Nottingham, on Saturday morning.
Unlike those in London, Mr Farage was not calling for another referendum, but for Brexit to take place, saying the delay allowed by the EU this week was an "outright betrayal".
He added that "if there was another referendum we would win it by a much bigger margin.
"I think people have seen what Europe's really like, they've seen the bullying, but just the gap that's opened up between the people and the politicians, it's a real problem."
The day of Remain and Leave demonstrations come at the end of a week when EU leaders agreed to delay Brexit to give Prime Minister Theresa May a final chance to get her deal through Parliament.
Anti-Brexit protesters have gathered in Madrid to voice their concerns around the UK leaving the European Union
Under the plan set out at the EU summit, leaders agreed to extend Brexit to May 22 if Mrs May can finally get MPs to back her deal in a third Commons “meaningful” vote.
However, if she fails the UK will have to set out an alternative way forward by April 12, which could mean a much longer delay – with the UK required to hold elections to the European Parliament – or leaving without a deal at all.
It had been expected that a third meaningful vote would be held on Mrs May's Brexit deal next week, but on Friday she hinted she may pull the vote.
In a letter to MPs, the Prime Minister admitted she may not garner enough support to get her twice-defeated Brexit deal through the Commons next week, an outcome which would further undermine her power, leading some to suggest she could not survive as Prime Minister if this was to occur.
A petition calling on the Government to halt the Brexit process had passed four million signatures on Saturday morning.