A million protesters are estimated to be marching through central London demanding a second referendum on Brexit, organisers have said, in one of the biggest demonstrations to be held in the UK in the 21st century.
The estimate, provided by the People's Vote UK, would make it the biggest march to be held in the UK since the Iraq War protest in 2003.
The anti-war demonstration also saw around a million people march in central London.
"#Breaking: 1 million estimated to be on the #PutItToThePeopleMarch," said Chuka Ummuna, newly independent MP for Streatham and one of the founders of the People's Vote campaign.
Mariella Frostrup and Richard Bacon, who hosted a rally during the march in Parliament Square, told the crowds an initial count showed the amount of people taking part had topped one million.
Anna Soubry, who is affiliated to the newly-founded Independent Group along with Mr Ummuna, was filmed cheering in reaction to the news.
"We've just heard how many people have come along to the #PutItToThePeopleMarch in London today," a spokesperson for The Independent Group said on Twitter.
"Incredible support from people along the route too. Thank you."
Anti-Brexit protesters began walking from Park Lane to Parliament Square on Saturday morning.
A group of activists from a group called the "Left Block" kicked off the march by unfurling of a large banner over Westminster Bridge that read “Love socialism, hate Brexit”.
In Parliament Square, Labour's deputy leader Tom Watson is expected to tell demonstrators that the only way to resolve the Brexit gridlock is "for people themselves to sign it off".
Other speakers will include former Conservative cabinet minister Justine Greening and ex-attorney general Dominic Grieve, newly independent MP Anna Soubry, Lib Dem deputy leader Jo Swinson, Green Party MP Caroline Lucas and SNP Westminster leader Ian Blackford.
On Saturday in Madrid, around 100 British expats also rallied against Britain leaving the EU.
The demonstrations follow EU leaders agreeing to delay the UK's exit from the organisation, giving Theresa May a final chance to get her withdrawal deal through parliament.
Leaders agreed that Brexit will go into effect on 22 May, if the prime minister can get MPs to back her deal in a third vote on the issue.
It has already been rejected by MPs in two votes on the issue and on Saturday The Times newspaper reported that pressure was growing on the prime minister to resign.
If the deal fails to pass once again, the UK will have to set out an alternative way forward by 12 April, which could mean a much longer delay - with the UK required to hold elections to the European Parliament - or leaving with no deal.
An online petition asking the government to stop Brexit process topped four million signatures by Saturday morning.
It is now the most popular ever submitted to the UK parliament website, moving ahead of a 2016 petition calling for a second EU referendum.
Additional reporting by agencies
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