Shadow chancellor John McDonnell said: "By trying to deny MPs a meaningful vote on their Brexit deal May and Raab have blundered into turning this issue into a fundamental question of parliamentary sovereignty and democracy in our country".
Joe Trickey, from Croydon, is celebrating his 83rd birthday at the march.
"Going out puts us in isolation and leaving isn't about trade deals, it's about our values".
Who has backed the protest?
Several MPs and celebrities have confirmed they will be attending including Alan Partridge star Steve Coogan, TV chef Delia Smith, actor and comedian Dawn French, Tory MP Anna Soubry, Green Party's Caroline Lucas and The Mayor of London Sadiq Khan.
Simon Chater is part of the Devon for Europe group, which arranged eight coaches to bring campaigners to London for the event.
The 69-year-old from South Devon said: "This is the first time in my life I've been political".
"The idea that you should have a second referendum would be incredibly damaging - most of all to the trust in democracy from people up and down this country", Richard Tice, founder of the opposing Leave Means Leave campaign, said.
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Prime Minister Theresa 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.
According to the Evening Standard, MPs are very keen for people to turn up and make their voices heard: representatives from the three major parties all stressed the importance of making the march a success.
It has caused further turmoil and growing anger within the ruling Conservative Party ranks, with most pro-Brexit ministers opposed to any such extension.
One of the works was called Bridge Over Troubled Brexit Waters and depicted Mrs May carrying a cross over a river of bad Brexit headlines.
The campaign, "March for the Future", from Park Lane to Parliament Square was dubbed by the organisers, Open Britain, as the "biggest, loudest and most important" demonstration on Britain's future.
It will then march to Parliament Square to hear speeches in support of a People's Vote from 2pm.
Miss Longman, 20, said she was four months too young to vote in the referendum.
She said: 'We're both Spanish students due to study overseas next year, but no one knows what will happen with Erasmus funding'.