LONDON — Lara Spirit, an anti-Brexit organizer, was studying politics at the University of Cambridge in 2017, reading about the use of referendums as a way of settling political disputes. It was timely coursework: A year earlier, Britons had voted to leave the European Union.

But Ms. Spirit was growing alarmed at the omission of young people from the Brexit debate. Despite warnings in her readings about the risks of referendums, she delayed school and founded a group with other students to push for another public vote on Brexit.

They called it Our Future, Our Choice, and on Saturday it will be among a clutch of youth groups helping to lead an expected crowd of hundreds of thousands in a march on Parliament in support of a second referendum.

“We were sort of horrified that there hadn’t been a youth group that was a voice on the issue of Brexit,” Ms. Spirit, 22, said. “It will be my generation, of course, who will be making sense of this and dealing with the consequences of the decision we took in 2016.”