Hundreds of thousands of supporters of the European Union marched through London on Saturday in the biggest demonstration so far to demand that the British government holds a public vote on the terms of Brexit. The protesters waved the blue and gold flag of the EU and held up "Bollocks to Brexit" banners under sunny skies to call for another referendum on the eventual deal on how Britain will leave the world's biggest trading bloc.

The march comes after another tumultuous week for Prime Minister Theresa May in which she failed to agree a divorce deal with EU leaders in Brussels and infuriated members of her own party by making further concessions in the talks. With just over five months until Britain is due to leave there is no clarity about what a future trade deal with the EU will look like and some rebels in May's Conservative Party have threatened to vote down a deal if she clinches one.

James McGrory, one of the organizers of the march, said voters should have the chance to change their minds because the decision will impact their lives for generations, Reuters reported. "People think the Brexit negotiations are a total mess, they have no faith in the government to deliver the promises that were made, partly because they cannot be delivered," he said.

At the march, demonstrators carried placards saying "Brexit is pants", "Time for an EU turn" and "European and proud."

Organizers said about 700,000 people took part in the march, which would make it the largest in Britain since a demonstration against the Iraq war in 2003. The "People's Vote" campaign, which includes several pro-EU groups, said they had stewards stationed at regular intervals to estimate the size of the crowd. The police did not provide an independent estimate of numbers participating. Protesters originally gathered near Hyde Park and then walked past Downing Street and finished outside parliament where they listened to politicians from all main political parties.

Britain is scheduled to leave the EU on March 29, but negotiations over the divorce have been plagued by disagreements, particularly over the future border between the Republic of Ireland and Northern Ireland. It will be the U.K.'s only land border with the EU after Brexit, for Ireland is part of the EU and Northern Ireland is part of the U.K. One of the great accomplishments of the 1998 peace deal that ended decades of violence in Northern Ireland was to dismantle the police and military presence at the border with Ireland. Many on both sides do not want a "hard" border again. There are also growing fears of a "no-deal" British exit, which could create chaos at the borders and in the EU and the British economies.

The prime minister has repeatedly ruled out holding a second referendum. May, speaking at an inconclusive EU summit in Brussels this week, said she would consider extending a proposed 21-month post-Brexit transition period for the U.K — one that could keep Britain aligned to EU rules for more than two years after its March departure. The EU has said extending that period would give more time to strike a trade deal that ensures the Irish border remains friction-free. Pro-Brexit politicians in Britain, however, saw it as an attempt to bind the country to the bloc indefinitely.