Demonstrators have begun arriving in central London to join a march on Parliament to demand a second Brexit referendum.
Hundreds of thousands of people are expected to pour into the capital on Saturday to take part in the Put it to the People march.
The day’s activities were kicked off by the unfurling of a large banner on Westminster Bridge that read “Love socialism, hate Brexit”.
The stunt was organised by a group calling itself the “Left Bloc” which is supported by Labour MPs, including Clive Lewis and Kate Osamor, Green Party MP Caroline Lucas, trade unions and grassroots campaigners.
— Clive Lewis MP (@labourlewis) March 23, 2019
Saturday’s march will see protesters process from Park Lane to Parliament Square, where a rally will be held.
As many as a million people could flood the streets of London after estimates for a similar rally in October were as high as 700,000.
In Parliament Square, Labour’s deputy leader Tom Watson is expected to tell marchers that the only way to resolve the Brexit impasse is “for people themselves to sign it off”.
Scottish First Minister Nicola Sturgeon, former deputy prime minister Lord Heseltine and London mayor Sadiq Khan are also expected to take the stage.
— Nicola Sturgeon (@NicolaSturgeon) March 23, 2019
Other speakers will 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 are arriving to the capital from across the country, with one taking on a 715-mile journey on ferries, trains and buses from Orkney in Scotland.
Student Sorcha Kirker, 27, will be joined by about 30 other students from the University of the Highlands and Islands.
The London march coincides with pro-Brexit campaigners continuing their long hike from the North East to the capital.
Former Ukip leader Nigel Farage re-joined the March to Leave when it set off from Linby, near Nottingham, on Saturday morning.
Saturday’s demonstrations follow EU leaders agreeing to delay Brexit to give Prime Minister Theresa May a final chance to get her deal through Parliament.
Leaders agreed to extend Brexit to May 22 if Mrs May can get MPs to back her deal in the Commons at the third time of asking.
If the vote is not passed, 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.
An online petition demanding the Government stops the Brexit process had topped four million signatures by Saturday morning.