Protesters calling for a referendum on the final Brexit deal have gathered in London for what organisers said would be the "biggest, loudest and most important" demonstration of its kind.

A march was due to start at noon before a rally in Parliament Square, organised by the People's Vote campaign.

Young voters are leading the march, which the organisers are expecting to be more than 100,000-strong.

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

MPs from all the main political parties are supporting the demonstration.

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.

'Chaos and confusion'
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.

The current plan is for a transition period of 21 months to smooth the path from Brexit to the UK and EU's future permanent relationship. But with the two sides failing to reach a deal so far, it was revealed this week the arrangement could be extended.

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.

"It's a mess that nobody voted for and the reason we're in such a growing crisis is that those cheerleading for Brexit know the promises they made can never be kept."

But 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."

He added that the demonstrators "just need to accept they lost".

'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 at midday.

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

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."

#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.

BBC journalist Charlotte Gallagher, with the demonstrators on Park Lane, said there were many families there with young children, a lot of whom were draped in EU flags.

Bea, 14, who travelled from Norwich with her mum Emma and brother Richard told her: "I'm marching because they're ruining our country, leaving it to my generation to sort out the mess."

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."

Kimberley Gray, 25, was born in County Fermanagh and now lives in West Sussex. She said she was marching for families there as she's worried about the peace process and a possible hard Irish border.

'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'."

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