LONDON — They marched by the hundreds of thousands to stop Brexit. They mourned when Prime Minister Boris Johnson walloped their side in the latest election.

And now, on the precipice of Brexit, ardent pro-European Britons, who by some measures outnumber those favoring Britain’s withdrawal from the European Union, are looking for new outlets for their rage — something, anything, to keep from staring into the abyss of a generation-long exile from the bloc.

“I don’t know what to do anymore,” said Tanya Luker, standing in the corner of the Coach & Horses pub in central London this week. “We’re just like a group of unhappy people that don’t know what to do.”

On Friday, Brexit will pass from the realm of left-wing nightmare to reality. And Britain’s Remainers, who fought for years to stay in the European Union, are reckoning with how to keep afloat a movement that may not get another chance to reverse Brexit for 30 or 40 years.