Nicola Sturgeon has called for a lengthy extension to allow a second Brexit referendum ahead of the Put It To The People march in London.
The Scottish First Minister urged opponents of Brexit to seize the "moment of maximum opportunity" presented by the delay agreed by the EU.
Before she joined the rally backing a second EU referendum, Ms Sturgeon said: "This is now the moment of maximum opportunity - we need to avoid both the catastrophe of no deal and the damage which would be caused by the Prime Minister’s bad deal."
Ms Sturgeon said: "The EU’s decision to postpone things until at least April 12 has opened a window, and those of us who oppose Brexit must seize the chance it offers.
"The House of Commons must now take back control from Theresa May and secure a longer extension to the process, to allow time for a new referendum to be held."
She added: "The Prime Minister and her government have proved completely incapable of delivering on the result of the 2016 vote, which is why it is right that this should now go back to the people."
Ms Sturgeon also argued Scotland, which as a region had a majority of remain votes, has been "completely ignored by the UK Government throughout the Brexit process".
Ms Sturgeon added: "That has demolished Tory claims that the UK is a partnership of equals, and is making the case for independence stronger than it has ever been.
"But whatever Scotland’s future – and I hope and believe it will be an independent one – it is in all of our interests for UK to be in the EU."
The Put It To The People rally will have figures from across the political spectrum taking part.
Ms Sturgeon will speak alongside Conservative peer Michael Heseltine and former Attorney General Dominic Grieve and Labour’s London Mayor Sadiq Kahn as well as MPs Jess Phillips and David Lammy.
Edinburgh South Labour MP Ian Murray said: "I will be marching on behalf of the 78 per cent of voters in Edinburgh South who backed remain.
"This is a march to stop the madness of Brexit.
"The last 1,000 days have destroyed trust in politics, and time is now running out to avoid a catastrophic no-deal exit from the EU."
"This national crisis can only be resolved by putting the decision back to the British public.
"It’s heartening that political figures are putting aside their differences for this, and it’s particularly encouraging that Nicola Sturgeon is now willing to share a platform with a Tory."
Scottish Liberal Democrat leader Willie Rennie added: "From the hundreds of thousands marching across the country to the millions of people signing petitions and speaking out, I have rarely seen the public so engaged with politics.
"The country is at a perilous crossroads and people are deeply concerned. However, Theresa May and Jeremy Corbyn still refuse to act in the national interest."