This article is more than 11 months old

This article is more than 11 months old

In one of the largest public demonstrations in British history, a crowd estimated at around one million marched outside parliament to demand MPs grant them a fresh referendum on Brexit.

Organisers of the march said the turnout, buoyed by the dry weather and the promise of “super Saturday”, was comparable to that of a second referendum rally six months ago when a million people gathered in central London.

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A spokesperson for the People’s Vote campaign said: “Our assessment is based on professional advice, there can be no doubt that this ranks as one of the greatest protests this country has ever seen.”

As MPs debated the future of the country inside Westminster, huge crowds assembled in Parliament Square and chanted for the British people to have a final say.

Led by the London mayor, Sadiq Khan, protesters from all corners of the UK had earlier gathered in Hyde Park, their numbers bolstered by many EU nationals living here, including a group of 50 pro-Catalan independence protesters.

Aerial footage showed people marching 40 abreast down Whitehall towards Westminster, a portion of central London completely swamped as the crowd filled all side streets along the route.

Many waved EU flags and placards stating “Together for the final say”, the title of the protest. Others held effigies of Boris Johnson, while one group pulled a float carrying a figure of senior Downing Street aide Dominic Cummings – with “Demonic Cummings” daubed across its forehead – using the prime minister as a puppet.

Shortly before 3pm, when news that MPs had voted to back the Oliver Letwin amendment broke, the thousands crammed in Parliament Square erupted wildly and began shouting “people’s vote”.

Later, the Labour MP David Lammy revealed that he could hear the crowd from inside the Commons. “We can hear your roar,” he tweeted.

As the celebrating crowd continued to spill into the streets outside Westminster, Khan told them that EU citizens were a vital element of what made London a great city.

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“They are our friends, members of our family, they are our colleagues. Over the last three years they have been feeling anxious, worried and heartbroken,” he said. “I want you to look around. This is what democracy looks like.”

However, the upbeat atmosphere soured after the MPs’ vote when prominent the Brexiters Andrea Leadsom and Jacob Rees-Mogg, walking with his son, were aggressively heckled by protesters despite a heavy police presence.

The estimated size of the march means that only the 2003 protest against the Iraq war, which some estimates put at as high as 1.5 million people, was definitively larger.

Organisers said the exact number was impossible to calculate but they had used “flow and fill rates and crowd density monitoring” to calculate its approximate size. The Metropolitan police said they would not be offering crowd size figures, but at one point issued a statement saying the crowd was so dense that marchers might not even be able to reach Whitehall, let alone parliament.

From the outset the numbers seemed to match predictions of a vast turnout, with many marchers saying they were kept stationary at Hyde Park Corner, close to the march’s starting point, and “severe bottlenecks” reported along Piccadilly.

The march organisers are also asking people to sign a letter to Boris Johnson, EU leaders, MPs and MEPs, asking them to allow “the chance to check whether we want to proceed with Brexit”. By Saturday night more than 23,000 people had signed it.