Activists want a fresh referendum in which Britons can have a say on the final Brexit deal. London Mayor Sadiq Khan joined students, families, health workers and scores more for Saturday's march, which ended with a rally at Parliament Square.
Among the protesters was "Lord of the Rings" actor Andy Serkis, who said he believes there should be a second referendum "now that people are more informed" compared to 2016, when Britons voted to pull out of the EU.
He says "the will of the people doesn't have to stand still, it's not an immovable thing that is fixed."
London Mayor Sadiq Khan has joined thousands of voters calling for a second Brexit referendum at a major demonstration.
Khan said Saturday's event was a "march for the future" for young Britons, including those who were too young to vote in Britain's 2016 EU membership referendum, when those who favor leaving the bloc won by 52 percent to 48 percent.
The mayor, from the opposition Labour Party, has previously backed mounting calls for a fresh referendum so that the public can have a say on whether they accept Conservative Party Prime Minister Theresa May's Brexit deal or choose to stay in the EU.
He told the BBC that "what's clear is that the only options on the table now from the prime minister are a bad Brexit deal, or no deal whatsoever. That's a million miles away from what was promised two-and-a-half years ago."
Thousands of protesters are gathering in central London to call for a new referendum on Britain's departure from the European Union.
Organizers want the public to have a final say on the government's Brexit deal with the EU, arguing that new facts have come to light about the costs and complexity of Britain's exit from the bloc since Britons voted to leave in 2016.
London Mayor Sadiq Khan from the opposition Labour Party was among those set to address the People's Vote March on Saturday, which will culminate at a rally in Parliament Square.
Prime Minister Theresa May of the Conservative Party has ruled out another public vote on the subject.
Britain is scheduled to leave the EU on March 29, but negotiations have been plagued by disagreements, particularly over the issue of the land border between the Republic of Ireland and Northern Ireland border.