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.

Best signs from the Put It To The People protest Show all 50 1 /50 Best signs from the Put It To The People protest Best signs from the Put It To The People protest A cheerful reminder Angela Christofilou/The Independent Best signs from the Put It To The People protest I will always love EU EPA Best signs from the Put It To The People protest Naughty Angela Christofilou/The Independent Best signs from the Put It To The People protest Bridget Jones vox populi Angela Christofilou/The Independent Best signs from the Put It To The People protest Anyone fancy a spot of birdwatching? Reuters Best signs from the Put It To The People protest Not the only one EPA Best signs from the Put It To The People protest 800sqm banner of a David Davis quote in Parliament Square Led By Donkeys/PA Best signs from the Put It To The People protest Does anyone? Angela Christofilou/The Independent Best signs from the Put It To The People protest Prime concern Angela Christofilou/The Independent Best signs from the Put It To The People protest Managing well to keep their wigs on Angela Christofilou/The Independent Best signs from the Put It To The People protest Name a more moronic duo. I'll wait EPA Best signs from the Put It To The People protest Is telekinetic intervention the only way out of the Brexit mess? Tom Bell Best signs from the Put It To The People protest The sane voice of youth Angela Christofilou / The Independent Best signs from the Put It To The People protest epaselect epa07458575 Protesters with a banners 'You don't speak for Me' attend the 'Put it to the People' march in London, Britain, 23 March 2019. Hundreds of thousands of people take part in the protest calling for a referendum on the final Brexit deal. EPA/NEIL HALL NEIL HALL EPA Best signs from the Put It To The People protest Needs must Angela Christofilou / The Independent Best signs from the Put It To The People protest Angela Christofilou/The Independent Best signs from the Put It To The People protest The scarier sequel Angela Christofilou / The Independent Best signs from the Put It To The People protest 'Tis only a flesh wound EPA Best signs from the Put It To The People protest Put the kettle on Reuters Best signs from the Put It To The People protest What's scarier - a shark attack or Brexit? EPA Best signs from the Put It To The People protest An artistic take Reuters Best signs from the Put It To The People protest A cryptic one Angela Christofilou / The Independent Best signs from the Put It To The People protest As a matter of fact EPA Best signs from the Put It To The People protest A windy one Angela Christofilou / The Independent Best signs from the Put It To The People protest A pioneering solution Angela Christofilou / The Independent Best signs from the Put It To The People protest Analogies EPA Best signs from the Put It To The People protest A joyful image EPA Best signs from the Put It To The People protest Generation Z priorities Reuters Best signs from the Put It To The People protest You're never too young to march for change PA Best signs from the Put It To The People protest A cheesy line PA Best signs from the Put It To The People protest The number one single Getty Best signs from the Put It To The People protest Quite the point Angela Christofilou / The Independent Best signs from the Put It To The People protest Do you? Angela Christofilou / The Independent Best signs from the Put It To The People protest Allegedly Angela Christofilou / The Independent Best signs from the Put It To The People protest Let's be considerate PA Best signs from the Put It To The People protest Something smells foul Angela Christofilou / The Independent Best signs from the Put It To The People protest Stuck in the middle with EU Reuters Best signs from the Put It To The People protest Decked out Angela Christofilou / The Independent Best signs from the Put It To The People protest Is it too late? Reuters Best signs from the Put It To The People protest Ikea's set the bar high Getty Best signs from the Put It To The People protest This picture puzzle is pretty darn clear Reuters Best signs from the Put It To The People protest Witty wordplay EPA Best signs from the Put It To The People protest Wonder Woman's battle cry for unity Reuters Best signs from the Put It To The People protest No one can question the wisdom of Spock PA Best signs from the Put It To The People protest The younger generation has spoken AP Best signs from the Put It To The People protest Throw us a bone here AP Best signs from the Put It To The People protest Canine stay in the EU? AP Best signs from the Put It To The People protest Put It To The People protest 23 March 2019 Angela Christofilou/The Independent Angela Christofilou/The Independent Best signs from the Put It To The People protest Put It To The People protest 23 March 2019 Angela Christofilou/The Independent Angela Christofilou/The Independent Best signs from the Put It To The People protest A demonstrator holds a placard during a Peoples Vote anti-Brexit march in London, Saturday, March 23, 2019. The march, organized by the People's Vote campaign is calling for a final vote on any proposed Brexit deal. This week the EU has granted Britain's Prime Minister Theresa May a delay to the Brexit process. (AP Photo/Tim Ireland) Tim Ireland AP

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".

Scottish First Minister Nicola Sturgeon 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, 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.

Support free-thinking journalism and attend Independent events

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.