According to The People's Vote campaign, the march is "the biggest demonstration on Brexit yet" and the second biggest demonstration this century behind the 1 million estimated to have protested against the Iraq War in 2003.
The Mirror spoke to two students leading the march behind a People's Vote banner.
Stand-up comedian Eddie Izzard donned a navy suit and waved Union Jack and European flags as he strolled through the capital's streets.
'Both these scenarios are a million miles from what was promised.
The march, which organizers said drew more than 600,000 to west London's Park Lane, was called by People's Vote, a group that's campaigning for the the public to have a say on any final Brexit deal to prevent the ruling Conservative government from forcing it through Parliament.
Mr Longworth said he believes Theresa may will not be able to get a second referendum happen through Parliament and that "she certainly will not get it through without the help of Labour, which would be unprecedented". Everybody needs to know this'.
James McGrory, one of the organizers of the march, told Reuters the public should have the chance to change their minds because the decision will impact their lives for generations.
Grassroots campaign group People's Vote UK, said: "This Government has failed on Brexit: there is no mandate for its vehicle crash proposal or for a disastrous no-deal Brexit".
Some chanted "People's vote!" Police did not provide an attendance estimate.
Co-chair of Southampton for Europe group Charlotte Sunley said: "I am marching because I believe that given the reality of what a deliverable Brexit actually means for our society, a majority of people in the United Kingdom now want the country to stay in the European Union".
The household names were also joined by TV and radio presenter Mariella Frostrup and Sadiq Khan, who took to the stage and addressed the crowds.
The protesters waved the blue and gold flag of the European Union and held up "Bollocks to Brexit" banners under sunny skies to call for another referendum on the eventual deal on how Britain will leave the world's biggest trading bloc.
A no-deal Brexit could create chaos at the borders and in the economy.
May said at an inconclusive European Union summit in Brussels this week that she would consider a longer post-Brexit transition period - one that could keep Britain aligned to European Union rules and obligations for more than two years after its March departure. See PA story POLITICS Brexit Protest.
"This week's fresh chaos and confusion over Brexit negotiations has exposed how even the best deal now available will be a bad one for Britain", Labour Peer Andrew Adonis said.
Organizers handed out postcards for people to write to their local lawmakers demanding a final say on the Brexit deal, which remains under negotiation between the United Kingdom government, led by Prime Minister Theresa May, and the European Union.