A new campaign group, backed by a star from the Star Trek movies, was launched Sunday calling for a public vote on a final Brexit deal.
James McGrory, executive director of Open Britain, which is supporting the People's Vote campaign, said: "Whether you think the government will negotiate a good deal or bad deal, Brexit is definitely a big deal".
Organisers said about 1200 people were at the event, including MPs from all leading parties.
Sir Patrick claimed the characters in his most famous screen roles, Star Trek's Captain Jean-Luc Picard and Charles Xavier from the X-Men, would have supported Britain's European Union membership.
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".
When asked whether it would take super powers to change the situation, the 77-year-old continued: "The two roles I am most well-known for, Jean-Luc Picard from Star Trek and Charles Xavier from X-Men, were excellent, admirable individuals - yes, intellectuals, but also compassionate and concerned for the wellbeing of everyone". And why? Because unity, common cause, well-being of society and debate were paramount to belief of this fictional character. 'Our country's future is at stake and we will not stand idly by, ' he said.
The actor, who last January said it was "embarrassed to be British" in a tweet about Brexit, also said he was motivated by "history and emotion" to want to stay in the EU.
The organisation wants the public, not politicians, to have the power to approve or reject the final Brexit deal and held a rally in London yesterday.
And Green Party co-leader Caroline Lucas told attendees that Brexit was "not inevitable", adding: "We'll do everything we can in Parliament for a people's vote. They would have voted Remain".
Appearing alongside Mr Umunna on ITV's Peston on Sunday, she said: "A second referendum, Chukka, which is what you're really campaigning for, is never going to happen".
The Star Trek actor told the BBC's Andrew Marr Show that the "terms and conditions" of Brexit were "quite unlike" how they were presented during the run up to the 2016 referendum.
Sir Patrick went on to discuss his upbringing in working-class Yorkshire, where he witnessed the aftermath of the Second World War.
Foreign Secretary Boris Johnson countered with a Star Trek allusion of his own, promising that Britain's departure from the European Union will allow it "to boldly go again to areas that perhaps we've neglected over the last 45 years".