The People's Vote march is due to start at 12pm today, as tens of thousands of people will walk from Pall Mall to Parliament Square as remainer groups continue to demand a second referendum on the final deal that is agreed with Brussels. But the country - and its Conservative government - remain divided about what kind of economic relationship it wants with the EU.
Lib Dem leader Sir Vince Cable, Green co-leader Caroline Lucas, Labour former NEC member and actor Tony Robinson and pro-EU campaigner Gina Miller were among those who joined the crowd in the capital on the second anniversary of the European Union vote.
The People's Vote estimated that 100,000 took part, but the figure could not be independently verified. Police did not give an official estimate.
There were boos from the crowd as the march approached Downing Street.
People's Vote March Crowds.
On senior cabinet members warning the United Kingdom was prepared to walk away from talks with Brussels, he said: "This is megalomania".
The protest is part of a "summer of action" by campaign groups created to increase pressure on Theresa May and the opposition Labour Party leader Jeremy Corbyn.
"Parliament is fiddling at the margins while the country slowly burns", he is expected to warn as he calls on the Government to vote on the deal, or no deal, with the option of staying in the EU.
"But two years later, all we've got are broken promises, an economy that's already feeling the strain of Brexit and a government paralyzed by internal divisions", he said.
Victoria Lewis, 55, a maths teacher from Brighton who wore a blue T-shirt that read "Citizen of Europe", came out to rally "because I have three children and they need to be part of Europe".
Neither of Britain's two main political parties back the idea of holding a referendum on the final deal.
Rejecting claims the Government has failed to make adequate preparations for a no-deal Brexit, he told the paper: "There's lots going on, we haven't made it public for very simple reasons".
Sir Vince is expected to say Brexit is not a "done deal" or inevitable and can be stopped.
Sixty former cabinet ministers, MPs, economists and business figures signed a letter to the Prime Minister urging her to issue orders to departments to accelerate planning for Britain to operate under World Trade Organisation (WTO) rules if a deal can not be done. Others, including Treasury chief Philip Hammond, want to keep closely aligned to the bloc, Britain's biggest trading partner.
Labour MP for Tottenham David Lammy said Boris Johnson's remarks were "no longer humorous" after the Foreign Secretary wrote in The Sun about the public not wanting "some bog roll Brexit, soft, yielding and seemingly infinitely long".
Fox told the BBC it was "essential" the European Union "understands. and believes" the Prime Minister's assertion that no deal would be better than a bad deal.
May's team is about to enter into the next round of negotiations with European Union counterparts, but is still to define exactly what it wants from Britain's future relationship with the continent, particularly in the area of customs regulation. A paper setting out the U.K. government position on future relations, due to be published this month, has been delayed until July because the Cabinet can not agree on a united stance.
A number of major businesses, including banks and auto makers, have said that a failure to reach free-trade agreement could disastrous for them.
This week, global trade secretary Liam Fox insisted the May was "not bluffing" when she says the government is willing to walk away from negotiations without a deal.
Airbus Commercial Aircraft chief operating officer Tom Williams said any Brexit scenario had "severe negative consequences" for the United Kingdom aerospace industry, Airbus in particular.