Hundreds of thousands of demonstrators turned out for the People's Vote march in London, demanding a final say on any Brexit deal between the UK and the EU.
Organisers said 700,000 people marched in the capital's gridlocked streets on Saturday, with some protesters saying the march had turned into more of a shuffle.
Participants finished in Parliament Square, where politicians and celebrities addressed the crowds, after setting off from Park Lane.
Speakers included Tory MP Anna Soubry, Labour's Chuka Umunna and Lib Dem leader Sir Vince Cable.
"It is clear we are the many," said Ms Soubry addressing supporters. "We are winning the argument, most importantly against those who voted Leave."
Taking aim at those who spearheaded the Leave campaign, she asked: "Where are they?" And added: "We will take responsibility and sort out this mess."
Mayor of London Sadiq Khan also addressed the crowds as several celebrities turned out for the event and posted photos online of the march.
In a video message, Scottish First Minister Nicola Sturgeon said her MPs would back a people's vote - "which includes the option to remain in the EU" - if the issue came before the House of Commons.
"The Tory government's handling of these negotiations has been chaotic, incompetent and shambolic," she said.
"Having spent two years telling us that no deal was better than a bad deal, the prime minister is now preparing to pile pressure on MPs to vote for a bad or blindfold deal on the grounds that 'no-deal' would be catastrophic.
"She is trying to scare the UK into the frying pan out of fear of the fire. It is a scandal and it should not be accepted."
Among the famous faces at the rally was Lord Of The Rings star Andy Serkis, who called it "one of the most, if not the most important march of a generation".
Others included presenter Richard Bacon, Dragon's Den star Deborah Meaden and TV cook Delia Smith.
Speaking to Sky News, Ms Soubry was delighted with the turn out at the event but admitted it did not equate to the 17 million people who voted Leave.
She said: "The thing that's really striking though is that when we started this, it was truthfully at our opening event back in April with two or 300 people.
"Then we marched in June and it was 100,000 people, and today it looks like it's at least 700,000 and I think that shows the movement, the fact we are winning the argument and most importantly it's among people who had voted Leave.
"Leave voters are now looking at Brexit and they are beginning to appreciate the complexities of it. They are not happy at all, this is not what people voted for - the chaos, the mess, the uncertainty."
One of those Leavers is 43-year-old Jason Gillot, from London, who changed his mind five days after the referendum.
"I'm politically agnostic but I was just sick of the lies that have come out of both sides," he said, adding that he chose to vote Leave because of "economic evidence partly supplied by the Tax Payers' Alliance" which he said "made sense at the time".
While the People's Vote march got under way, a pro-Brexit rally got took place at the same time in Harrogate, North Yorkshire.
Supporters of leaving the EU were addressed by Nigel Farage, and MPs Kate Hoey and Nigel Owen Paterson in a rally that pledged to "rescue Brexit" and express "support for the Brexit Britain voted for".
The event, run by Leave Means Leave and thought to be attended by around 1,200 people, branded the campaign for a second referendum a "losers' vote".