London, United Kingdom – Hundreds of thousands of people have marched in central London to demand a new referendum on whether Britain should remain in the European Union, as parliament voted to delay a decision on whether to back Prime Minister Boris Johnson‘s revised Brexit withdrawal deal.

The march organised by the People’s Vote campaign is thought to be the largest yet, drawing anti-Brexit supporters from across the country as parliament sat on a Saturday for the first time since the Falklands conflict in the 1980s.

Protesters struggled to make their way out of jammed underground stations to join the march, which set off as customary from Hyde Park to make its way down to Trafalgar Square and on to the Houses of Parliament.

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They held placards saying “I’m 17 and Brexit stole my future” and “UK and Northern Ireland at peace not in pieces”, referring to Johnson’s deal, which sets out a revised plan for Northern Ireland, a major stumbling block in the previous withdrawal agreement negotiated by his predecessor, Theresa May.

Two previous marches have attracted between half a million and a million people. A record number of coaches – more than 170 – were booked to ferry protesters from across the country, some of them sponsored by local celebrities.

As thousands of EU flags fluttered in the streets of the capital – an unthinkable sight before the referendum that divided the UK in 2016 – legislators voted for an amendment designed to prevent the UK from crashing out of the bloc on October 31 without a deal.

MPs voted 322 to 306 in favour of the amendment, which effectively prevents Johnson’s Brexit deal being approved until parliament has signed off on the necessary legislation to implement the deal.

Johnson responded to the defeat by doubling down on his previous pledge to take the UK out of the EU by the existing October 31 deadline. He promised to bring the withdrawal agreement before MPs next week.

“The day is extremely significant,” Femi Oluwole, cofounder of Our Future Our Choice (OFOC), a youth advocacy group part of the People’s Vote campaign, told Al Jazeera.

Protesters struggled to make their way out of jammed underground stations to join the march [Ylenia Gostoli/Al Jazeera]

He believes a second referendum to be the only way to “resolve the Brexit mess”.

“If you want a specific answer on Brexit, then you have to ask the specific question,” he said.

“A general election wouldn’t actually solve anything,” Ludo Sappa-Cohen, a spokesperson for another anti-Brexit group taking part in the march on Saturday, Best for Britain, explained.

“It’s quite likely it would end up in a hung parliament, then you’ve got the same gridlock in Westminster as you have today. It would be divisive, not decisive. A second referendum is much more likely to unlock political progress. It makes sense, three years down the line, to give people the final say.”

Simon Usherwood, deputy director of the academic think-tank UK in a Changing Europe, explained that the Letwin amendment effectively “avoids parliament falling into a situation where it gave an approval to the deal, but then it couldn’t guarantee legislation would be done in time for the October deadline.”

“As well as having this vote, the UK needs to pass a withdrawal agreement bill, which nobody has seen the text of, and which will have major constitutional implications,” Usherwood told Al Jazeera.

“The government still doesn’t have a majority in Parliament. The chances that piece of legislation could get through by the end of the month look relatively small, even with the pressure [of a possible no-deal outcome].”

Sally Patterson, a 23-year-old student walking down to Westminster, said: “The country is in chaos and no one voted for this kind of mess.”

“I think what we need is a people’s vote because no one knows what is happening any more. What people voted for four years ago now, it’s not necessarily what they believe now with all the facts on the table,” Patterson, who is a campaigner with the student group For Our Future’s Sake (FFS), added.

“Even if that vote ends up [to be in favour of] leaving the EU, at least we’ll know that’s what the people want.”

Simon Blandel, a 53-year-old retired teacher, said he wanted to be part of the EU. “Although Europe isn’t perfect, you can’t change it without being in it,” he said.