More than half a million anti-Brexit campaigners gathered to march in central London calling for a second referendum dubbed 'The People's Vote March'.
The march set off from Park Lane at midday and ended in a rally in Parliament Square, London Mayor Sadiq Khan and celebrity chef Delia Smith are among those who spoke at the event.
Organisers, who originally expected around 100,000 expected to attend, said 670,000 demonstrators marched the streets.
About 150 buses ferried thousands of activists from across the country to the capital in Britain's biggest protest since the Iraq War.
Thousands of campaigners arrived dressed in royal blue clothing emblazoned with gold stars, brandishing signs which read 'bin Brexit now' and 'I want a say on Brexit'. Leading speeches in Parliament Square, Delia Smith said people were not fully informed when they voted but now understood 'the dire consequences'. 'When the vote first happened we weren't fully informed,' she said.Organisers of the rally, People's Vote UK, posted to Twitter: 'Well over HALF A MILLION are marching on Parliament today, demanding a #PeoplesVote on the Brexit deal. Everybody needs to know this.'
The protesters were joined by other famous faces including comedian Eddie Izzard, who came dressed in a navy suit waving British and EU flags. Mr Khan said Saturday's event was a 'march for the future' for young Britons, including those who were too young to vote in Britain's 2016 EU membership referendum, when those who favor leaving the bloc won by 52 percent to 48 percent.
The mayor, from the opposition Labour Party, has previously backed mounting calls for a fresh referendum so that the public can have a say on whether they accept Conservative Party Prime Minister Theresa May's Brexit deal or choose to stay in the EU.
He told the BBC that 'what's clear is that the only options on the table now from the prime minister are a bad Brexit deal, or no deal whatsoever. That's a million miles away from what was promised two-and-a-half years ago.' He added: 'I can't think of anything more democratic, anything more British, than trusting the judgement of the British people.
James McGrory, one of the organisers of the march, said voters should have the chance to change their minds because the decision will impact their lives for generations.
'People think the Brexit negotiations are a total mess, they have no faith in the government to deliver the promises that were made, partly because they cannot be delivered,' he said.
At the march, demonstrators carried placards saying 'Brexit is pants', 'Time for an EU turn' and 'European and proud.' Organisers said about 670,000 people took part in the march, which would make it the largest in Britain since a demonstration against the Iraq war in 2003.
The 'People's Vote' campaign, which includes several pro-EU groups, said they had stewards stationed at regular intervals to estimate the size of the crowd. The police did not provide an independent estimate of numbers participating.
Meanwhile, Former UKIP leader Nigel Farage joined Brexit supporters in Harrogate for this afternoon's Save Brexit rally at Harrogate Conference Centre.It's one of four being held around the country organised by Leave Means Leave.
Britons voted to leave the trade bloc by a narrow margin in the 2016 EU membership referendum and Prime Minister Theresa May has ruled out another public vote on the subject.
Britain is scheduled to leave the EU on March 29, but negotiations have been plagued by disagreements, particularly over the issue of the Republic of Ireland-Northern Ireland border, which will be the UK's only land frontier with the EU after Brexit. There are growing fears of a 'no-deal' exit, which could create chaos at the borders and in the economy.
Prime Minister Theresa May said at an inconclusive EU summit in Brussels this week that she would consider a longer post-Brexit transition period - one that could keep Britain aligned to EU rules and obligations for more than two years after its March departure.
Pro-Brexit politicians in Britain saw it as an attempt to bind the country to the bloc indefinitely.This week's fresh chaos and confusion over Brexit negotiations has exposed how even the best deal now available will be a bad one for Britain,' said Andrew Adonis, a Labour member of the House of Lords.
'Voters will neither forgive nor forget if (lawmakers) allow this miserable Brexit to proceed without people being given the final say.'
Simon Chater is part of the Devon for Europe group, which arranged eight coaches to bring campaigners to London for the event.The 69-year-old from South Devon said: 'This is the first time in my life I've been political.'
He said 400 people had travelled in coaches arranged by the group, with some leaving Devon at 6am.
Campaigners have begun the march to Parliament Square, led by a group of young voters calling for a second referendum.
Emma Stevens and Emily Longman are two of the students leading the march behind a People's Vote banner.Miss Longman, 20, said she was four months too young to vote in the referendum.
She said: 'We're both Spanish students due to study abroad next year, but no one knows what will happen with Erasmus funding.'
Miss Stevens, also 20, said: 'We don't want the other European countries to hold the same view [of leaving the EU].'
