“The message is clear: stop Brexit,” said the leader of the Liberal Democrats Vince Cable, who led the vast crowd which set off at lunchtime ahead of a rally outside parliament.

“We are now a Remain country. Nearly 90 per cent of young voters who weren’t allowed to vote in 2016 would vote to remain,” he added.

In Westminster speakers including Scotland’s First Minister Nicola Sturgeon, London Mayor Sadiq Khan and opposition Labour deputy leader Tom Watson began addressing attendees from early afternoon.

A host of other political figures also joined the demonstration, including Independent Group MPs who quit Britain’s two mainstream parties – the Conservatives and Labour – earlier this year in protest at their Brexit policies.

“One million marching for Britain’s place in Europe – a worthy fight,” said Chris Leslie, one of the breakaway lawmakers. A majority of MPs voted against holding another nationwide poll on the issue earlier this month, and it remains a long shot bid by Remainers.

But with the protest coming just days after EU leaders granted a delay to Brexit, people are increasingly hopeful they could still force a public vote.

“I was too young to vote in the last referendum,” science student Calypso Latham, 19, told the BBC.

“It’s going to affect my career with research grants so I definitely wanted to come and protest.”

This week’s agreed delay to Brexit, which was set for March 29, has prompted Prime Minister Theresa May to make a renewed bid to win MPs’ backing for her divorce deal.

If she succeeds, Britain will depart on May 22 under the terms of the withdrawal agreement the prime minister struck with Brussels last year.

But if lawmakers defeat the accord again, as expected, London must outline a new plan or face a no-deal Brexit as early as April 12 – unless it decides to request another extension and hold European Parliament elections in May.

Hosted by the People’s Vote pressure group, Saturday’s protest follows a similar demonstration in October that drew an estimated half a million people.

Organisers have arranged hundreds of coaches and even chartered a train to bring protesters from all corners of the country to the capital.

Stephen Goodall, a 96-year-old second world war veteran, travelled 300km by train from southwest England alongside four generations of his family including his great-granddaughter.

“I am an old man and the outcome won’t affect me – but it will affect my family and many people that I know for years to come,” he said ahead of the event.

The prime minister has repeatedly ruled out holding another poll on the issue, claiming it would be divisive and renege on promises to honour the 2016 referendum result.

Meanwhile the main opposition Labour Party appears divided on the issue.

At its 2018 conference, it backed holding another poll as a last resort, while advocating staying in a customs union with the EU together with close alignment with its single market.

But some MPs are fierce advocates of putting it back to the people, while others representing Leave-supporting areas in central and northern England, are bitterly opposed.

In a sign of the splits, at least half a dozen Labour shadow ministers are set to join deputy leader Tom Watson at Saturday’s march, while the party itself asked activists instead to help campaign for local elections due on May 2.

Watson said he had now decided to campaign for a referendum “reluctantly” and would back May’s deal if it was also put to the people.

“It can only begin to bring the country back together again if we all have a final say – and then live with the result,” he said.

“I trust the people I represent. And only they can sort this mess out.”