Organizers of Saturday’s “Put It To The People” march in London claim close to a million people showed up to protest against Brexit. As the possibility of England exiting the European Union without a deal on April 12 looms, even that number may not be enough to turn heads.

M.P. Anna Soubry, who recently left the Conservative party, urged her fellow M.P.s to “put your country first,” and support a true “people’s vote” to reexamine Brexit. But the idea of another vote is generally unpopular among lawmakers.

London Mayor Sadiq Khan also added his voice to the growing chorus for a second referendum, telling assembled crowds in Parliament Square that “Brexit is a complete and utter mess.” The Labour party’s deputy leader Tom Watson, Scottish First Minister Nicola Sturgeon, and former deputy prime minister Michael Heseltine also spoke at the rally.

Heseltine criticized Prime Minister Theresa May in particular, for this week’s suggestion that members of parliament were deliberately undermining her, before throwing his support behind another vote. “One way or another, you the people must decide. You the people must be free to vote to remain,” he said.

By Saturday afternoon, a parliamentary petition to “Revoke Article 50 and remain in the EU” inched closer to 4.5 million signatures. The BBC also reports that Liberal Democrat MP Layla Moran claimed the petition might “give oxygen” to the campaign for another Brexit referendum.

As it stands, Theresa May is due to deliver a third potential Brexit deal to Parliament next week, though she may scuttle the proposal if it does not find enough support among the M.P.s. Approving May’s deal would also extend the Brexit deadline until May 22. Otherwise, the U.K. is due to leave on April 12. May delivered a letter to Parliament on Friday, which outlined four “clear choices,” including approval of the deal, asking for another extension (in which case, the U.K. would take part in elections for the European Parliament), revoking Article 50, or leaving the E.U. with no deal.

Despite the growing protest, in her letter May suggested that revoking Brexit outright would be to “betray the result of the referendum.”