Demonstrators have begun marching through central London to demand a second Brexit referendum.
Hundreds of thousands of people have poured into the capital to take part in the Put it to the People march.
Sadiq Khan tweeted a video of the countdown launching the Put it to the People march, with the London mayor holding up a banner at the front of demonstrators.
He wrote: ‘And we’re off!
‘Here in London, thousands of people from across our city and country have come together with @peoplesvote_uk to send a clear message: Enough is enough – it’s time to give the British public the final say on Brexit. #PeoplesVote #PutItToThePeople.’
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.
The march sees 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 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.
Other speakers will include former Conservative cabinet minister Justine Greening and ex-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 arrived in 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 demonstration follows 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 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 this morning.
It is now the most popular ever submitted to the Parliament website, moving ahead of a 2016 petition calling for a second EU referendum.
Leader of the Liberal Democrats Sir Vince Cable has tweeted his support for a second referendum after launching the march.
Alongside two photos of him with demonstrators, he wrote: ‘Great to kick off the #PeoplesVoteMarch just now.
‘There is a huge turnout of people here from all walks of life, of all ages and from all over the country.
‘We are a Remain country now with 60% wanting to stop the Brexit mess. #ExitfromBrexit #PutitToThePeople’