Organisers claim more than 500,000 anti-Brexit campaigners have joined a central London protest calling for a fresh referendum.
The People's Vote march set off from Park Lane and is expected to end in a rally in Parliament Square.
Celebrity cook Delia Smith, Dragons' Den businesswoman Deborah Meaden and London Mayor Sadiq Khan are among the speakers at the event, which is being compered by broadcasters Richard Bacon and Mariella Frostrup.
— Chuka Umunna (@ChukaUmunna) October 20, 2018
The People's Vote campaign said its stewards estimated the size of the crowd to be around 570,000.
Sadiq Khan, the Mayor of London, speaking ahead of his platform appearance, said: "Today will go down as an historic moment in our democracy. A moment when in their thousands, people from every corner of our country and every section of our society will take to the streets – coming to make our voice heard.
"We've heard some complain that a public vote would be undemocratic and unpatriotic. But the opposite is true. There's nothing more democratic – nothing more British – than trusting the people to have the final say on our future.
"The lies, the mistruths and the deceptions of the referendum campaign have now been exposed, and it's clear the will of the people is changing. No one voted to leave the EU to make themselves poorer. No one voted to make life harder for our children and grandchildren. No one voted to see our NHS damaged. And no one voted for the shambles that this Government has created."
— People's Vote UK (@peoplesvote_uk) October 20, 2018
In a video message of support, Nicola Sturgeon, the First Minister of Scotland, said: "Let me say this loudly and clearly, if the issue comes before the House of Commons, SNP MPs will support a People's Vote which includes the option to remain in the EU."
She added: "The Tory government's handling of these negotiations has been chaotic, incompetent and shambolic.
"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."
Delia Smith said people were not fully informed when they voted but now understood "the dire consequences".
"The only way we can avoid this total madness and win back our future has to be a People's Vote."
The march was led by a group of young voters calling for a second referendum.
Emily Longman, 20, said she was four months too young to vote in the referendum.
Miss Longman, who was marching alongside Emma Stevens, also 20, said: "We're both Spanish students due to study abroad next year, but no one knows what will happen with Erasmus funding."
Joe Trickey, from Croydon, was celebrating his 83rd birthday at the march.
He said: "I believe very strongly in the EU as a place of peace and strength.
"Going out puts us in isolation and leaving isn't about trade deals, it's about our values."