An estimated 1 million anti-Brexit protesters rallied alongside leading politicians in central London on Saturday to demand a second referendum on the U.K.’s withdrawal from the European Union, Trend reported citing Bloomberg.

Marchers clogged the streets of central London as they walked from Hyde Park to Parliament Square to hear from the opposition Labour Party mayor of London, Sadiq Khan, Scottish First Minister Nicola Sturgeon and deputy Labour leader Tom Watson.

Watson promised to back Theresa May’s twice-defeated political deal -- breaking from the party’s position -- in return for her agreement to put the withdrawal accord to a public vote.

“I will support your deal, I will help you get over the line, to help avoid a disastrous no-deal Brexit, but only if you let the people vote on it,” Watson said.

Mariella Frostrup, an organizer, said more than a million people had joined the protest that spread from outside Houses of Parliament and along Whitehall to Trafalgar Square, where large-screen TV screens were set up for overflow crowds.

Protesters in favor of Brexit also took to the streets, though in a far smaller number. About 100 protesters clad in bright yellow vests -- similar to the French Gilets Jaunes movement -- were given a police escort as they chanted “Brexit now.”

Kahn, calling himself a “proud European,” said May had squandered the good will of EU leaders with a confused approach to the withdrawal agreement.

“No matter which political party you support, we can all agree that Brexit has been a complete and utter mess. The British people didn’t vote for the government to gamble on the future,” he said.

Watson’s pledge to back May goes further than Labour leader Jeremy Corbyn’s offer. Corbyn has proposed letting May’s deal pass in return for a second referendum, but only by abstaining, a condition which would require her to muster more support from her own divided party. Corbyn remains wary of alienating the sizable proportion of Brexit-supporting Labour voters.

“If it was 52-48 percent again, I don’t think I could go along with that at all,” said marcher Katie Overstall, 27, when asked about Watson’s proposed public vote. “It would have to be a more significant majority.”

The march follows a tumultuous week for May. The premier had sought a three-month extension from the EU, but got only a two-week delay to April 12. If lawmakers approve her deal on a third attempt, the U.K. will have until May 22 to negotiate an orderly exit. There’s no guarantee May can bring the agreement to the House of Commons for a third vote, after it was blocked in principle by Speaker John Bercow.

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A similar protest in October drew an estimated 700,000 people. Saturday’s marchers will be accompanied by live performances from noted U.K. musicians including DJ Fatboy Slim.

Separately, an online petition seeking to stop Brexit by revoking the Article 50 notification collected more than 4.28 million people by midday Saturday, the most names ever collected in a public campaign.

The previous record for an online drive was 4.15 million names in 2016 on a proposal to hold a second Brexit referendum, according to the Parliament website.