LONDON — Tens of thousands of protesters in London set off on Saturday afternoon from Hyde Park to Parliament in a last-ditch effort to reverse Britain’s looming split from the European Union, in a sign of the enormous anger among pro-Europeans in Britain about the stalemate among lawmakers.
The chances that the huge crowd snaking through the streets of London, carrying placards and the European Union flag, would persuade lawmakers to back a second referendum on Britain’s withdrawal, known as Brexit, are remote. But that did not stop a petition calling to cancel Brexit from racking up more than 4.2 million on Parliament’s website on Saturday — the same day as the march.
“This is the first time I’ve felt that I needed to come and take part,” said Jenny Chandler, 54, a food writer from Bristol, arriving under the Victorian arches of London’s Paddington Station before setting off on the march.
“I’m feeling disempowered and frustrated, and even though it feels slightly futile, I wanted to be here today,” Ms. Chandler said. “It’s our last glimmer of hope to stay in the E.U.”
Protesters around her clutched coffees and waved European Union flags, their chatter drowned out by the rumble of trains. Ms. Chandler wore a blue beret with gold stars, a nod to the European Union flag, that she had borrowed from a neighbor.
The idea that the British could still be debating in late March a reversal of the coming split from the bloc once seemed far-fetched. The country had been scheduled to leave on March 29. But Parliament remains deadlocked. The left and right despise Prime Minister Theresa May’s plan.
And even as deadlines were pushed back and European leaders preached patience with their counterparts in London, the prospect of a calamitous no-deal Brexit looms closer every day.
The organizers of the march on Saturday say there is one way out of the impasse: Give Britons another chance to vote on whether to leave the European Union. The country voted by a 4 percent margin to extract itself from the bloc in a referendum in 2016.
A crowd expected to number in the hundreds of thousands began converging on London on Saturday morning. They came by bus and train, posting pictures and videos on Twitter, many of them young people who described the fight for another public vote on Brexit as the defining political event of their lives.
[Read: Young People Are Leading the Campaign for a Second Brexit Vote]
“Hello, and welcome aboard the People’s Vote express from Bristol Temple Meads to London Paddington,” Sally Patterson, 23, an officer at the University of Bristol Students’ Union, said into a microphone aboard a train on Saturday, before handing it back to the conductor.
Another organizer asked people in a video posted on Twitter to walk to Coach F to make placards.
Some of the estimated three million European Union citizens living in the country, many of whom say they resent not having been given a chance to vote on the issue, were expected to take part, as well.
The demonstration was a stark contrast to an ongoing march “against Brexit betrayal.”
A few dozen Brexit supporters — backed by Nigel Farage, the former leader of the euroskeptic U.K. Independence Party — set out on March 16 in Sunderland, in northeast England, a community that voted in favor of Brexit.
They paid 50 pounds each for kit and accommodation along the route, and have been walking south toward London ever since, headed for a rally on March 29 where organizers hope many more Brexiteers will join them.
Hanging over the anti-Brexit march in London was the reality that a second referendum remains unlikely. Opposition Labour Party leaders, who have long dithered over a second referendum, have never energetically backed the idea.
There was also the question of how a pro-European campaign tainted in 2016 by its association with the political elite of London could create a new message three years later, one that appealed to smaller towns and cities in the English North and Midlands.
Yema Stowell, 25, who works for an academic publisher in Oxford, England, and campaigned to remain in 2016, said Brexit had drawn her into organizing in a way that traditional party politics had not.
“When the campaign started, I thought this is something I believe in — political parties not so much,” she said.
But she said people were fast losing hope. She said an aunt who had a dress matching the European Union flag had decided not to join the march on Saturday. Still, she said, whether a second referendum was ultimately held, it was important to speak up for Britain’s relationship with Europe.
“This is about telling Theresa May that the people aren’t on her side,” she said, referring to comments this past week by Mrs. May in which she said lawmakers were undermining the public with their attempts to stall her exit plan.
After arriving on a packed train to London, Ms. Stowell walked out of Paddington Station to join a stream of people making their way toward Hyde Park.
Another protester, Olivia Leydenfrost, 58, a British-American communications professional who grew up in Manhattan but has lived in Britain for 17 years, had arrived in London from Bath. She said the right-wing movements in Britain and the United States had motivated her to march.
“Because I’m a U.S.-U.K. citizen, I have Trump back there and Brexit here, and these two things have made me become an activist,” Ms. Leydenfrost said.
She said Mrs. May’s efforts to hold her fractured Conservative party together, despite the clock ticking down to a no-deal Brexit, were difficult to watch. “What is devastating is the Tories willing to sacrifice the country for the party,” she said.
Protesters have repeatedly gathered outside Parliament since the 2016 referendum, carrying messages that included support for the country’s membership in the European Union and demands for a hard exit from the bloc.
As Mrs. May repeatedly failed to persuade enough lawmakers to back her plan, there have been signs of growing support for the idea of a second referendum, though pollsters’ findings have varied widely depending how the question is asked, and backing in Parliament remains sparse.
Mrs. May’s recent trip to Brussels led to an extension of the Brexit deadline, giving British lawmakers until April to approve her deal. If it is rejected, the options include a chaotic British exit without an agreement with the European Union or a longer extension of the deadline.
The most likely form a second referendum would take is a choice between the prime minister’s exit deal and remaining in the European Union. Many polls show that Britons have now gone from mostly thinking that Britain was right to leave Europe after the referendum to mostly thinking the opposite.
Support for a public vote has also grown, according to a mid-March poll by YouGov for the People’s Vote campaign, with the option of staying in Europe favored over Mrs. May’s divorce deal or a no-deal exit. But the latter prospect has also grown more popular among Brexit supporters.
In October, a march for a second referendum gathered what the organizers said was nearly 700,000 protesters in London. A British newspaper later obtained a local government estimate that put the crowd at a still-considerable 250,000 people.