The demonstrations were held as EU leaders gathered in Rome to mark the 60th anniversary of the founding agreement of the union
Tens of thousands of people took to the streets in London this afternoon to protest against Brexit – just days before the expected triggering of article 50 and the beginning of Britain’s formal separation from the EU.
The The Unite for Europe demonstration began with the crowds observing a minutes silence in tribute to the victims of Wednesday’s attack at Westminster.
Organisers said more than 25,000 people took part in the march from Park Lane in Westminster to Britain’s Houses of Parliament.
Police were forced to delay the start of the march as buses brought in crowds from around the country.
Liberal Democrat politician Nick Clegg told the crowd that it is time for “sadness” about the Brexit referendum result to give way to “a perpetual sense of anger about the choices that Theresa May and her government have taken since.”
“It was a choice to pull us out of the customs union, it was a choice to embark on that demeaning bout of transatlantic obsequiousness,” said Mr Clegg
He also accused the British Prime Minister of “threatening to turn our country into a bargain basement cowboy economy.”
Journalist and former Downing Street press advisor Alastair Campbell told the crowds that while he may be in a minority in thinking Brexit can be stopped, “I am not in a minority in thinking that it should be."
“When you see a car heading toward a cliff, you don’t keep driving,” he told the crowds.
“The media in this country is a right-wing cartel of tax-dodgers that pretends to speak for their readers when they speak for themselves and their own vested interest.”
Liberal Democrat leader Tim Farron said the march was about showing “solidarity and respect for those who voted leave.”
He said Mrs May, “does not speak for 52%, she barely speaks for 5%.”
“We are not giving up this week of all weeks,” he said. “We here are as testament that we refuse to despair. Britain can be better than this.”
There was also a smaller anti-Brexit protest march in Scotland where up to 1,000 protesters took to the streets of Edinburgh.
Protestors gathered in the city centre and marched towards the Scottish Parliament.
Jean Francois-Poncet, the chairman of the Young European Movement Edinburgh said there were “lies in the referendum campaign that people were not held accountable for and - whether you voted Remain or Leave - that is a real issue.”
Britain voted to leave the EU in a referendum in June 2016.
The marches coincided with a number of demonstrations throughout Europe - as EU leaders gathered in Rome to mark the 60th anniversary of the founding agreement of the EU.
European Commission President, Jean-Claude Juncker told the gathered leaders that it was a “tragedy” that member states were celebrating the 60th anniversary of the Treaty of Rome without Britain.