The People's Vote campaign wants a referendum so the public can ratify or reject the final Brexit agreement, due to be concluded between London and Brussels in October, before Britain leaves the European Union on those terms at the end of March 2019.
More than a thousand people packed into the Electric Ballroom venue in north London for the raucous launch, complete with t-shirts, flags, posters and pints.
Lawmakers from the governing Conservative Party, as well as the opposition Labour, Liberal Democrat and Green parties, and "Star Trek" actor Patrick Stewart, addressed the crowd.
"It's a vote to ratify the final deal, which is different to the initial thoughts of people when they bought this product," Lib Dem MP Layla Moran, one of the speakers, told AFP afterwards.
"This isn't just for Remainers. This is for Leavers too."
Race against time
After Britain leaves the EU, a transition period will run until the end of 2020.
The transition deal will not be finalised unless Britain and the other 27 EU countries agree on divorce terms by October - Brussels' deadline to wrap up talks.
The British government wants to quit the single market and customs union, while aiming to retain the closest possible ties with the bloc.
Moran said any new referendum would have to happen after the deal is completed in October, and before Brexit day at the end of March - meaning campaigners are short on time.
The first step would be to convince enough lawmakers - crucially Labour and Conservative backbenchers, emboldened by demand from the public - to get a new referendum in legislation, likely through an amendment to a Brexit bill.
MPs would then set the referendum question, which could potentially include several options: accepting the deal, rejecting it and leaving on World Trade Organisation terms, or staying in the EU.
To the launch's loudest cheers, Labour MP Chuka Umunna urged his party to be "true to its values" and support a new referendum, as people angrily asked why Labour leader Jeremy Corbyn was absent.
Brexit 'not inevitable'
The push is run by Open Britain, which grew out of the official Remain campaign of the 2016 referendum. The campaign brings together eight other anti-Brexit groups.
"Brexit is not inevitable. What the government comes back with, not what was promised in the referendum, will be the real deal. It should not be a done deal," said Open Britain executive director James McGrory.
"Brexit will affect everybody in the country, which is why it should not be left to 650 politicians to decide our future but 65 million people."
Brexit remains a fractious topic in Britain, with less than a year to go until the country quits the EU.
In the seismic June 2016 referendum on Britain's EU membership, 52 percent of the UK's British, Irish and Commonwealth residents, plus Britons abroad who had lived in Britain in the last 15 years, voted to leave.
