When British filmmaker Mike Leigh started work on his period political drama “Peterloo” five years ago, he made a firm decision to avoid superimposing modern-day political references onto the 200-year-old story.

But he overlooked one possibility: that political developments might impose themselves on his movie.

“From 2016 onward, in both the States and the U.K., there were events that resonate with the story,” he said. “I didn’t have to put in any contemporary analogies because people were going to be looking for them, anyway.”

Set in 1819, the film focuses on an effort by the British ruling elite to keep the working class — which was exploding in numbers because of the Industrial Revolution — in a subservient position by denying them the vote, levying taxes and imposing stiff import tariffs on basic necessities like the grain needed to make bread.

Tensions finally reached the breaking point during a massive protest in Manchester, held in St. Peter’s Field. Just four years after the Battle of Waterloo, it was labeled Peterloo by people who were adding the suffix “loo” to confrontations the same way we now append “gate” to scandals.

An estimated 60,000 people rallied to demand the right to vote. Armed soldiers were unleashed on the protesters, who were dressed in their Sunday finery and had been told not to bring anything even resembling a weapon to avoid the impression they were looking for a fight. They were helpless to defend themselves against the soldiers, who showed no mercy, cutting down the elderly, women and even children indiscriminately.

It’s a famous moment in British history. Although it’s largely unknown in the United States, Leigh said, viewers on this side of the Atlantic will have their own parallel reference points, including the 1965 attack in Selma, Ala., on demonstrators demanding voting rights for blacks.

“The Americans have Selma, the Irish have Bloody Sunday,” he said of a 1972 protest in which soldiers opened fire on civil rights marchers. Many democracies have an event like it in their histories, he said.

While the movie is neither a documentary nor a re-enactment, Leigh wanted it to be as historically accurate as possible. He did extensive research; so much so that the movie’s press kit includes three single-spaced pages of historical background. (By comparison, the plot synopsis takes up just half a page.)

“Some of the characters are composites, but they’re based on real people,” he said.

He also wanted to re-create the conditions of the rally as precisely as possible. To that end, most of the people amassed in St. Peter’s Field have no idea what the speakers are saying. At times, there are multiple people shouting over each other in a futile attempt to be heard.

“There were no PA systems then,” he said. “No doubt, when Abraham Lincoln gave the Gettysburg Address, no one standing more than 100 feet away from him heard a word of it.”

This is a different sort of project for Leigh, 76. For most of his career, he has preferred smaller, intimate dramas focused on personal relationships, like 1996’s “Secrets & Lies,” which won the BAFTA (the British Oscar) for best picture, the Palme d’Or at the Cannes Film Festival and the indie world’s Spirit Award.

This movie, on the other hand, is the very embodiment of the term “epic,” featuring 160 speaking roles plus an army of extras, and a running time that edges past 2 ½ hours.

“I’ve certainly never done anything on a scale like this,” Leigh said.

Nonetheless, he stuck with his traditional approach to filmmaking. He outlines a plot, casts the roles and then workshops the dialogue with the actors — all 160 of them, in this case.

It was a challenge, he admitted, but one he thinks was worth it.

The actors “did a lot of research on their own, and their performances were very informed by that,” he said. “I’m blessed that in the U.K. we have so many actors who were dedicated and committed to this.”