Similar events were held nationwide to launch an advertising campaign backed by nine groups including Open Britain.
Actor Patrick Stewart drew on his two best-known characters as he helped launch a campaign to give voters the final say on any deal Britain negotiates to leave the European Union.
Britain will leave the European Union in March 2019, and both the Conservative and Labour parties have said a second referendum will not take place.
However, both the Tories and Labour have ruled out the possibility of a referendum on the subject and time to change their minds is running out.
The People's Vote campaign is demanding the British public be given the final say on the deal secured by Prime Minister Theresa May, rather than MPs in Parliament.
Stewart said the campaign was not a rejection of the Brexit result, arguing that some of the claims used by the Leave side - including a promise of additional £350 million a day for the NHS - were "misleading".
Stewart, who portrayed psychic Professor Charles Xavier in the Hollywood series after finding global fame as the Star Fleet captain, appeared on the BBC's Andrew Marr Show yesterday as he prepared to launch the People's Vote campaign group.
"Brexit is not inevitable. It should not be a done deal".
An online poll from Opinium published to mark the launch suggests more than half of the British public (52%) backs a vote on the final deal, with 31% opposed.
During a speech to the crowd, Green Party MP Caroline Lucas will say: 'We will be making the case in Parliament, but this is too big and too important to be determined exclusively by politicians.
Chuka Umunna, MP for Lambeth, which had the highest Remain vote in the country, said: "We need more Conservative members of Parliament to be as courageous as Anna and many others who defied the whip and put their country before their party".
Liberal Democrat MP Layla Moran, a leading supporter of the People's Vote campaign, said:"It may seem like the odds are stacked against us as a movement, but fighting for what you believe in is never easy. Together, are on the verge of something extraordinary".
May has repeatedly said she will deliver what she calls the will of the people and on Sunday Foreign Minister Boris Johnson voiced optimism for the kind of deal Britain could win. "They would have voted Remain".