Thousands of pro-Europe protesters have flooded the streets of the capital, calling for the UK to strengthen ties to the Continent following the Brexit vote.
The March for Europe was launched to ratchet up pressure on the Government to delay activating the formal process of leaving the EU.
Campaigners marched from Hyde Park and through Whitehall to the Houses of Parliament, where a debate on whether a second EU referendum should take place will be held on Monday.
It comes after an online petition garnered more than four million signatures after the vote to leave the EU in June, but an official Government response to the campaign said the Brexit decision "must be respected".
The demonstrations are also calling for greater public consultation on every stage of the Brexit negotiations.
A number of activists and performance artists took to the stage to voice their support for unity with Europe, including left-wing journalist Owen Jones.
The author of The Establishment, who is not calling for a second referendum, said he felt it was important the Government was held to account in how it delivered Brexit.
He told the Press Association: "In a democracy you don't get a situation when one person wins and everybody else shuts up and has to support them, we've all got to use our freedom of expression to have a say in this country and that's what this (the rally) is about."
He added: "I think it's problematic arguing for a second referendum now. One side lost, relatively narrowly, but it was still a loss, you can't just have referendums until you get the right result.
"I think now the focus has to be on what the terms of Brexit currently are and that means people who voted remain having an input and being listened to, you don't have just one chunk of the country deciding our precise relationship with Europe and what Brexit means - all of us have to have that role as well."
Earlier, a confrontation had erupted in Whitehall when a cohort of Brexit campaigners yelled at the EU marchers as they passed each other in the street.
A group of men with hidden faces tried to block the passage of the march with a banner, but were moved aside by the blue-clad crowd.