Protesters calling for a referendum on the final Brexit deal are marching in London for what organisers say will be the "biggest, loudest and most important" demonstration of its kind.

They are heading to Parliament Square, where a rally will be held later.

Young voters are leading the march, which organisers the People's Vote campaign say is being attended by more than 500,000 demonstrators.

Prime Minister Theresa May has already ruled out such a referendum.

MPs from all the main political parties are supporting the demonstration. The People's Vote said stewards on the route estimated 570,000 were taking part.

Scotland Yard said it was not able to estimate the size of the crowd.

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Image caption Thousands arrived at the meeting point on London's Park Lane for the march

Meanwhile, former UKIP leader Nigel Farage is leading a pro-Brexit rally at Harrogate Convention Centre this afternoon, the latest in a series of events organised by the Leave Means Leave group.

The British public voted to leave the EU by a margin of 51.89% to 48.11% in a referendum in June 2016.

The UK is scheduled to leave on 29 March 2019, under the terms of the two-year Article 50 process.
Image copyright Reuters
Image caption A young boy holding a sign proclaiming support for the EU

Labour's Lord Adonis, a campaigner for People's Vote - which wants a referendum on the outcome of the Brexit negotiations - said: "Brexit's becoming a dog's dinner.

"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."
Image copyright EPA
Image caption Signs included "we want our country back" and "ask me again"

Richard Tice, founder of Leave Means Leave and former co-chair of Leave.EU, told BBC Breakfast: "The idea that you should have a second referendum would be incredibly damaging - most of all to the trust in democracy from people up and down this country."
'Final say'

Some 150 coachloads of people from across the UK - including as far away from London as Orkney - travelled to the March for the Future, which started in Park Lane.

Mayor of London Sadiq Khan - who started the march - is among those to speak at Parliament Square, along with representatives from the main political parties. Celebrity speakers will also include Steve Coogan, Delia Smith and Deborah Meaden.

Ms Smith said it was "the most important issue in our lifetime", adding: "My message to MPs is please sort this out. Let the people you serve have their say."
Media captionSadiq Khan on 'march for the future'

Mr Khan said: "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. In those circumstances, what we demand is that the British public gets a say in whether they accept the outcome of the negotiations, which could be a bad Brexit deal, it could be no deal, with the option of staying in the European Union."

First Minister of Scotland Nicola Sturgeon sent a message of support to the rally, saying the SNP would support a vote that would give the option of staying in the EU.
'Young stand to lose most'

#PeoplesVoteMarch was trending on Twitter on Saturday, with lots of young people - some of those who were not eligible to vote in the 2016 referendum - heading on the march.

Emily Longman, 20, one of the students leading the march behind a People's Vote banner, was four months too young to vote in the 2016 referendum.

She studies Spanish and is due to study abroad next year, but said "no-one knows what will happen with Erasmus funding".
Image caption Aleta Doyle and her son Leo travelled from Peterborough

Aleta Doyle, 46, from Peterborough, attending with her 12-year-old son Leo, said she was marching "for my children's future and European unity".

And Leo Buckley, 16, from Hampshire, said: "Young people stand to lose the most. I'm going to be poorer and not have the same career opportunities."
'Small print'

Dr Mike Galsworthy, from NHS Against Brexit, told BBC News: "We should retain control over what's going on.

"Whether you voted leave, or whether you voted remain - when a contract comes back, you do have the right to read the small print and say actually 'no, no. no, this isn't what we want to be signing up for'."

Campaign supporter Alastair Campbell, former Downing Street director of communications, said: "The Brexit that was promised, and the Brexit that was campaigned successfully for, doesn't exist."

He added: "I don't think you can re-run the referendum. I think we have to accept that we lost that debate. I think the question has to be on the nature of the deal."

It follows a march in London in June, on the second anniversary of the Brexit vote.