Protesters flying EU flags and holding banners with slogans including “Never going to give EU up” and “March for Europe” gathered in reaction to a referendum result that has plunged Britain into political chaos.
Though the UK voted narrowly by 52 per cent to 48 per cent to leave the EU, the capital, where the protest took place, strongly voted to remain – but is set to be dragged out of the bloc by the rest of the country.
Some of the Saturday morning crowd, many of whom were young adults, draped themselves in European flags while others painted their faces blue with yellow stars.
Marchers gathered in Park Lane at 11am and marched towards Parliament Square, via Picadilly, Trafalgar Square and Whitehall.
The protesters chanted “What do we want?" "EU!" as they set off for Westminster political district, with others singing "Hey EU" to the tune of The Beatles’ “Hey Jude”.
Many banners featured Tory Cabinet minister Michael Gove, who is standing for the Conservative leadership having campaigned for Brexit.
Others highlighted generational divides between Leave and Remain voters, with young people flying a "My generation will remain" banner and other placards suggesting "Our future has been stolen".
Organisers said 50,000 people turned out to the protest, while the Metropolitan Police estimated closer to 30,000 people. The closing rally at Parliament Square was completely full by 2.30pm, with reports of more protesters being unable to enter the square because it was at capacity.
From the stage set up in Parliament Square, Labour peer Lord Cashman told the crowd: "No more lies, no more hate. We need to uphold the values of democracy and inclusiveness which are at the heart of the EU and this country.
“We must not let right-wing, narrow-minded nationalism nor xenophobia define us. We are better than that. I honestly believe the disinformation in this campaign has undermined our democracy. Decent British values are also the values of the European Union.”
"You cannot deny geography. The UK is in Europe. How can you take it out?" he said.
A similar rally earlier this week in Trafalgar Square was cancelled due to heavy rain – but tens of thousands of people turned up anyway.
Britain voted to leave the European Union in a referendum last week after a long campaign fraught with ill feeling on both sides.
Support in urban areas and among young people was significantly lower, however. The capital strongly backed Remain by 60 per cent to 40 per cent.
The disparity between different parts of the country has promoted a four-million-signature petition calling for a second referendum and even a renewed push for Scotland to secede from the UK.
The event’s organiser, King’s College graduate Kieran MacDermott, wrote on the event's Facebook page: “We can prevent Brexit by refusing to accept the referendum as the final say and take our finger off the self-destruct button.
“It is the responsibility of Parliament to consider our democracy more carefully and call for a vote before they all accept the UK's decline.
Despite the vote, Britain will not immediately leave the European Union – with widespread disagreement among policymakers about how Brexit should happen.
Ukip leader Nigel Farage has said Article 50 should be invoked as quickly as possible, but former Mayor of London Boris Johnson, who also backed Leave, has said there should be no haste.
There is also disagreement, even amongst Leave campaigners, about what form Brexit should take. Whether Britain should remain in the European single market or control immigration is currently a matter of debate and was not settled at the binary referendum.
There have also been sportadic calls from some politicians and campaigners for a second referendum to confirm any deal.
EU officials and the leaders of other EU nations have called for Britain to move as quickly as possible towards a solution in order to minimise what is expected to be an economically difficult period of transition.