Politicians including John McDonnell and London Mayor Sadiq Khan, were due to address the crowds

Tens of thousands of pro-EU protesters from across Britain wielding banners, placards and flags converged on parliament on Saturday, as MPs debated the government's Brexit deal.

Beneath largely clear blue skies, demonstrators rallied near Hyde Park in central London before marching to Westminster to call for a second referendum on Britain's planned EU departure.

Walking behind a pink banner proclaiming "together for the final say", they chanted: "What do we want? People's vote! When do we want it? Now."

"The first referendum was jumping on a train without a destination," said Douglas Hill, 35, from Oxford, south central England, with his Estonian wife and their baby daughter.

"Now that we have a destination, we need to have a second referendum."

Another attendee, Theodor Howe, a 20-year-old student in Dundee, eastern Scotland, conceded another poll could be divisive but insisted it was still necessary.

"People should have a say in what is going to happen," he told AFP, expressing hope that MPs reject Johnson's deal and that he is forced to ask Brussels for another delay.

Politicians including John McDonnell, from the main opposition Labour party, and London Mayor Sadiq Khan, were due to address the crowds.

Organisers from the People's Vote pressure group laid on 172 buses to bring people to the British capital, with the cost covered by supporters from sport, business and entertainment, it said.

Former prime ministers John Major, a Conservative, and Tony Blair, of Labour, feature in a film set to be broadcast to the crowds once they arrive in Parliament Square.

"Whatever is the outcome, no deal or bad deal, it should not pass without the final say resting with the people," they said in a statement Friday.

London Mayor Khan was among those leading the crowd as it snaked its way towards parliament, which was holding its first Saturday sitting since the 1982 Falklands War as the British prime minister bids to win MPs backing.

The protesters unveiled an effigy depicting Johnson as a puppet operated by his chief advisor Dominic Cummings -- a highly divisive figure who led the 2016 campaign to leave the EU.

A smaller counter-protest by Brexit supporters draped in Britain's union flag was also staged in Westminster, with rival demonstrators verbally sparring with each other in the shadow in parliament.