London  - Anti-Brexit groups, led by the cross-party Open Britain, launched a campaign on Sunday for a "people's vote" on Britain's deal for leaving the European Union, urging the public not to let politicians decide the issue.

Pro-EU lawmakers - from Prime Minister Theresa May's Conservatives and the opposition Labour, Liberal Democrat and Green parties - spoke to hundreds of supporters at a rally in London to launch the People's Vote campaign.

"Brexit is a big deal, not a done deal, and it is too important for politicians alone to decide, which is why today we've launched a campaign for a People's Vote on the final Brexit deal," said Labour's Chuka Umunna, adding that he hoped his party would change its policy to back a new vote.

Actor Patrick Stewart was among several celebrities who attended the launch, while Liberal Democrat leader Vince Cable said he was "proud to support" the campaign.

People 's Vote Campaign

"As Lib Dems have long said, it should be the people, not politicians who have the final say on the Brexit deal," Cable said on Twitter.

Green Party co-leader Caroline Lucas responded to claims by opponents of a second referendum, who argue that another vote could subvert the democratic mandate from the first one in 2016, when 52 per cent voted for Brexit.

"Giving the people a chance to look at the deal is upholding democracy, not denying it," Lucas said.

"We will be making the case in parliament [for a vote], but this is too big and too important to be determined solely by politicians," she said ahead of the rally. "If the public demand a people's vote, politicians will fall in line," Lucas said.

Open Britain and other pro-EU groups staged what they called their largest ever national day of action on Saturday, with thousands of supporters attending more than 300 events across the country.

Re - referendum?

Tom Brake, the Liberal Democrat Brexit spokesman, said at one of Saturday's events that a new vote was crucial for younger voters who did not take part in the 2016 referendum.

"For our future's sake, it's really important that young people have a vote," Brake said.

May has repeatedly ruled out the possibility of a second referendum on Brexit, but she has promised to consult parliament on the final deal.