Thousands are gathering to march through central London calling for a second EU referendum, as MPs search for a way out of the Brexit impasse.
Demonstrators from the “Put It To The People” campaign will march from Park Lane to Parliament Square, followed by a rally in front of Parliament.
It comes after the EU agreed to delay the UK’s departure from the EU.
PM Theresa May has said she will ditch plans for another vote on her Brexit deal if not enough MPs support it.
Unless that deal is passed by MPs, the UK will have to come up with an alternative plan or else face leaving without a deal on April.
Meanwhile, a record-breaking online petition on Parliament’s website calling for Brexit to be cancelled by revoking Article 50 has attracted more than 4.18 million signatures.
Liberal Democrat MP Layla Moran said the petition could “give oxygen” to the campaign for another Brexit referendum.
Speakers at the rally include Labour’s deputy leader, Tom Watson, Scotland’s First Minister, Nicola Sturgeon, London Mayor Sadiq Khan, former Tory turned independent MP Anna Soubry and former attorney general Dominic Grieve.
Ms Sturgeon said now was “the moment of maximum opportunity” to avoid a no-deal Brexit.
Mr Watson is expected to promise to back Mrs May’s deal if she agrees to hold a referendum on it.
He is expected to say: “I’ve come to the reluctant view that the only way to resolve this and have legitimacy in the eyes of the public is for the people themselves to sign it off.”
Ms Soubry told the BBC: “It’s good to see Tom Watson coming on the march today… I honestly believe it’s the only way forward.”
She added: “It’s intolerable the situation that we’re in and I’m afraid Theresa’s the problem.
“I’m not saying the government should go because that’s the last thing we want, but I think she has to go and we need some temporary prime minister who can reach out, put the country first, get this back to the British people – that’s what we’re all marching for today, a people’s vote.”
Ms Soubry also said Mrs May had made a “desperately bad mistake” blaming the delay to Brexit on MPs in her televised address on Wednesday.
Former Tory chief whip Andrew Mitchell agreed “there was a very strong feeling she’d made an error of judgement” on this.
But dismissing reports of growing pressure on Mrs May to quit, he told BBC Radio 4’s Today programme: “To change prime minister would be a colossal error – it won’t change the numbers [in the vote for Mrs May’s deal]”.
If Mrs May’s deal is approved by MPs next week, the EU has agreed to extend the Brexit deadline until 22 May. If it is not – and no alternative plan is put forward – the UK is set to leave the EU on 12 April.
In a letter to all MPs on Friday evening, Mrs May offered to talk to MPs over the coming days “as Parliament prepares to take momentous decisions”.
Children’s minister Nadhim Zahawi told the Today programme that failing to support Mrs May’s deal would lead to a “meltdown in our politics, not just for the Conservative party but for all parties”.
He said all the other alternatives would require MPs asking for a much longer extension, which Mrs May has said she is not prepared for.
Indicating he would stand down if Mrs May’s deal is not voted for, Mr Zahawi said he “cannot justify” going to his constituents and saying: “We failed to deliver this and that now we are having to stay in the EU and go into European elections.”
The march comes as the pro-Brexit March to Leave, which started in Sunderland a week ago, continues towards London.
Former Ukip leader Nigel Farage re-joined the March to Leave in Linby, near Nottingham, on Saturday morning telling around 200 Brexit supporters that Mrs May had reduced the nation “to a state of humiliation”.
Speaking from the top of an open-top bus, Mr Farage said those gathering for the People’s Vote march in London were not the majority, before leading the crowds through the village.
Meanwhile, by 11:32 GMT on Saturday, the total number of signatures calling for Article 50 to be revoked stood at 4,151,815 – beating the previous record reached by another Brexit-related petition in 2016.
Margaret Georgiadou, who set up the petition, tweeted earlier that she had “received three death threats over the phone”, she also said she had closed her Facebook account after receiving a “torrent of abuse”.
Parliament’s petitions committee tweeted on Friday that the rate of signatures was “the highest the site has ever had to deal with”, after the website crashed.