Hundreds of thousands of people are expected to march on Parliament calling for the public to be given the final say on Brexit.
Demonstrators will flood into the capital from across the country for the Put It To The People march in Parliament Square on Saturday.
The number of marchers calling for a second referendum could soar to more than one million after estimates for a similar rally in October were as high as 700,000.
Campaigners will march from Park Lane to Parliament Square from midday, followed by a rally in front of Parliament.
There, Labour’s deputy leader Tom Watson will tell crowds that he has reluctantly come to the view that the only way to resolve the current impasse is “for people themselves to sign it off”.
Brexit is stuck in the parliamentary pipework. The impasse works for neither Leavers or Remainers. I have come to the reluctant view that the only way to resolve this is for the country to have the final say. Tomorrow I will join the #PutItToThePeople march. pic.twitter.com/78abwJzqHF— Tom Watson (@tom_watson) March 22, 2019
Mr Watson will tell the crowds in Westminster: “The current impasse is not working for people who voted to leave or people who voted to stay. I really don’t think Parliament will be able to resolve this.
“That’s why I’ve come to the reluctant view that the only way to resolve this and have legitimacy in the eyes of the public is for the people themselves to sign it off.
“It can only bring closure if we’re all involved in making the decision.”
Scottish First Minister Nicola Sturgeon, former deputy prime minister Lord Heseltine and London mayor Sadiq Khan will also address the protest.
Iâll be proud to speak at the @peoplesvote_uk march tomorrow. Scotland voted to remain in 2016 but people across the UK must have the chance to get out of this Brexit mess. Whatever Scotlandâs future - I hope independent - it is in all of our interests for UK to be in the EU.— Nicola Sturgeon (@NicolaSturgeon) March 22, 2019
Other keynote speakers include former Conservative cabinet minister Justine Greening and former attorney general Dominic Grieve, former Tory turned independent MP Anna Soubry, Lib Dem deputy leader Jo Swinson, Green Party MP Caroline Lucas and SNP Westminster leader Ian Blackford.
Campaigners will pour into London from across the country, with one embarking on a 715-mile journey on ferries, trains and buses from Orkney in Scotland.
Student Sorcha Kirker, 27, is the vice president for higher education of the Highlands and Islands Students’ Association, and studies archaeology at Orkney College UHI.
She said she will be joined by about 30 other students from the University of the Highlands and Islands.
She added: “Personally I think it’s really unfair because predominantly students and young people don’t want Brexit, and many were not old enough to vote in 2016, yet we are the ones whose futures it will affect for the longest.”
The march comes on the same day pro-Brexit campaigners will continue their long hike from the North East to London, with former Ukip leader Nigel Farage re-joining the March to Leave when it sets off from Linby, near Nottingham, on Saturday morning.
The day of Remain and Leave demonstrations come at the end of a week when EU leaders agreed to delay Brexit to give Prime Minister Theresa May a final chance to get her deal through Parliament.
Under the plan set out at the EU summit, leaders agreed to extend Brexit to May 22 if Mrs May can finally get MPs to back her deal in a third Commons “meaningful” vote.
However, if she fails the UK will have to set out an alternative way forward by April 12, which could mean a much longer delay – with the UK required to hold elections to the European Parliament – or leaving without a deal at all.
A petition calling on the Government to halt the Brexit process had passed three and a half million signatures on Friday evening.