At least 670,000 people turned out for the People's Vote march, organisers have claimed – making it the biggest anti-Brexit demonstration since the referendum in 2016.
Protesters told of virtual gridlock on the streets of London as they called for a final say on Brexit, joking that it had turned into more of a shuffle than a march.
The demonstration set off from Park Lane and finished in Parliament Square where celebrities and politicians including Conservative MP Anna Soubry, Labour's Chuka Umunna and Liberal Democrat leader Sir Vince Cable gave speeches.
— Chuka Umunna (@ChukaUmunna) October 20, 2018
Ms Soubry said: "It is clear we are the many".
Addressing the cheering crowds, she said: "We are winning the argument, most importantly against those who voted leave."
She added: "We will take responsibly and sort of this mess."
Mayor of London Sadiq Khan also addressed the crowd, while dozens of celebrities posted snaps of themselves on the protest on social media.
— 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."
Lord Of The Rings actor Andy Serkis attended the rally with his wife and son, and described it as "one of the most, if not the most important march of a generation".
Elsewhere, Crazy Rich Asians and Humans star Gemma Chan tweeted a snap of herself with a banner reading: "Even Baldrick had a f****** plan".
Great to see my local #Libdems controlled council Richmond Upon Thames flying the EU flag from the town hall in recognition of today's People's Vote March.Well done to Council Leader @Gareth_Roberts_ and his fellow @LibDems councillors for making this happen pic.twitter.com/Cb29mo4ND8
— Vince Cable (@vincecable) October 20, 2018
Other famous faces included TV presenter Richard Bacon, entrepreneur and Dragon's Den star Deborah Meaden, comedian Jenny Eclair and Holby City actors Catherine Russell and Hugh Quarshie.
Jason Gillot originally voted to leave the EU, but said he changed his mind five days after the referendum.
The 43-year-old, from London, told the Press Association: "I'm politically agnostic but I was just sick of the lies that have come out of both sides."
He said he initially voted to leave due to "economic evidence partly supplied by the Tax Payers' Alliance" which he says "made sense at the time."
"Now we have actual facts and realities of what's going to be happening," he added.
Mr Gillot has been marching with a sign that says: "When the facts change, I change my mind! What do you do sir?"
Joe Trickey, from Croydon, celebrated 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."
Mr Trickey was joined by his daughter, who is holding a sign about his birthday, and plans to celebrate afterwards with "a glass of wine".
Simon Chater – a member of the Devon for Europe group – helped arrange 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.
Emma Stevens and Emily Longman were among those holding the 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]."
Theresa May visited an arts exhibition in her constituency of Maidenhead while protesters gathered in London.
Titled Maidenhead And Me, the exhibition featured work by locals with different perspectives of the town.
The Prime Minister refused to answer questions about the fate of Saudi journalist Jamal Khashoggi and whether or not the UK would suspend arms sales to the Gulf state now it has confirmed Mr Khashoggi is dead.
The police refused to confirm the final figures for the march.