Protesters are staging a ‘people power’ rally to push for a referendum on the final exit deal.
Campaigners including actor Sir Patrick Stewart and politicians from across the political spectrum are demanding a second vote.
The People’s Vote campaign will demand voters are given the final say rather than Parliament on the deal secured by 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.
Sir Patrick told BBC One’s The Andrew Marr Show that the calls for a fresh vote were ‘part of democratic process’.
Asked what he would say to people who voted for Brexit, he told the programme ‘what we are doing is in their benefit’.
Sir Patrick said Jeremy Corbyn is a ‘puzzle’ to him after he was pressed on how the Labour leader had sacked shadow cabinet members who have differing views on Brexit.
Green 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.’