Opponents of Brexit are staging a "people power" rally to push for a referendum on the final exit deal.
Around 1,000 campaigners are expected to gather in London, including actor Sir Patrick Stewart and politicians from across the political spectrum.
The People's Vote campaign will demand voters are given the final say rather than Parliament on the deal secured by British Prime Minister Theresa May.
Liberal Democrat education spokeswoman Layla Moran will say: "It may seem like the odds are stacked against us as a movement, but fighting for what you believe in is never easy.
"We are short on time, but we have people power on our side. Together, we are on the verge of something extraordinary."
The campaign brings together nine grassroots campaigning organisations in a joint push for a vote.
Economist Mariana Mazzucato, Innocent Drinks founder Richard Reed and comedian Andy Parsons are also expected to attend.
Green Party MP Caroline Lucas will say: "We will be making the case in Parliament, but this is too big and too important to be determined solely by politicians.
"So, it's up to you. If the public demand a People's Vote, politicians will fall in line."
James McGrory, executive director of Open Britain - which is supporting the People's Vote campaign, said: "Whether you think the Government will negotiate a good deal or bad deal, Brexit is definitely a big deal.
"Brexit is not inevitable. What the Government comes back with, not what was promised in the referendum, will be the real deal. It should not be a done deal.
"Brexit will affect everybody in the country, which is why it should not be left to 650 politicians to decide our future but 65 million people. That is why so many are demanding a People's Vote on the final Brexit deal."