For two filmmakers who have, for much of their careers, been bracketed as twin bastions of British social realism, Mike Leigh and Ken Loach have never had much in common. Where Loach has long worn his liberal activist’s heart on his sleeve, Leigh tends to tuck his politics into the folds of his movies, national crises of class tension and inequality needling through intimate domestic scenarios. So it’s chief among several surprises in “Peterloo,” Leigh’s vast, many-headed, boomingly angry historical epic, that it plays almost as his version of a Loach film. Strenuously detailing the many small orders of business that led to Manchester’s devastating 1819 Peterloo Massacre, which saw the British Tory government ordering a brutal military charge into a working-class crowd of peaceful pro-democracy protesters, it’s a stately, explicitly rhetorical paean to the people — setting the past in stone while also lashing out at a present-day political order.

That makes “Peterloo” a frequently rousing experience, particularly as we slow-march toward the awful, eponymous event itself — staged with a stern, aggressive dynamism for which little in Leigh’s prior filmography has prepared us. But there’s no denying that the bulk of this 154-minute film is hard, stone-carrying work, bearing relatively little of Leigh’s signature human comedy, and swarming with characters and incidents that alert audiences must keep fixed in their heads, rather like students revising for an exam. That’s seemingly by design: Nothing about this grim passage of history was easy for its victims, and “Peterloo” sets out with textured integrity to honor the experience of those who suffered most under it, at whatever cost to its commercial arthouse appeal. Still, one wonders if the film has really found its optimum form: As Leigh diligently stretches to cover the tragedy from the perspective of every involved party, from royalty to rebels to the peasants trampled underfoot, “Peterloo” seems half-inclined to bust out of its already outsize cinematic corset into a full-blown miniseries.

Certainly, few films in recent memory have called as urgently for an old-fashioned dramatis personae at the outset: It’s telling that “Peterloo’s” press notes include a sprawling character tree, with players grouped under 11 different social substrata. (Perhaps audiences would benefit from such a handout.) Leigh wrongfoots us at the beginning, appearing to identify a central point of view in Joseph (David Moorst), the PTSD-stricken soldier son of a destitute family of Manchester mill-workers. A stunning opening shot circles him, dazed and crumpled and battle-stained, amid the carnage of Britain’s victory at Waterloo, before following him, in languid montage, on the long trudge home. Once there, however, he largely fades into the patchwork, his voiceless face reappearing — ever more hollowed-out, atop a progressively fraying military redcoat — at occasional key points in the action, a gaunt marker of the underclasses’ growing desperation.

The stricken lad’s retreat serves as a warning to viewers not to expect a protagonist from Leigh’s heaving societal panorama. His family, however, serves as its closest thing to a moral and narrative center, its members representing a range of grassroots responses to a burgeoning spirit of revolution. Infuriated by mounting post-war poverty, import restrictions and Parliament’s refusal to extend the vote to the working class, Joseph’s father Joshua (Pearce Quigley) begins attending meetings with change-minded radicals, as do his sons; his mother Nellie (a hard, hunched Maxine Peake) remains glumly complacent, skeptical that any good can come of resistance in her lifetime. “When has the government ever done anything to help us?” she asks, as grumbling ensues about the indifference of “fat leeches down in London.” Not for the first or last time, British viewers in particular should sense the Tory government of 2018 coming in for (barely) tacit censure here, as the working classes’ grievances in the film closely echo contemporary discourse around economic austerity and metropolitan elitism.

Meanwhile, action reverberating around the unrest somewhat justifies Nellie’s pause. Leigh’s focus drifts across the interlocked forces working against the Reformers: spies delivering reports from their meetings to the powers that be; ogre-ish, black-robed magistrates quelling their efforts with inordinate sanctions and economic restrictions; unfeeling Home Office lords and Sir John Byng (Alastair Mackenzie), the suave military hero they appoint as the Northern District commander despite (or rather because of) his complete lack of political interest; and the Prince Regent himself (Tim McInnerny), a clownish grotesque with no view past the rim of his own champagne glass, to whom the Reformers must nonetheless direct a futile petition for the vote. Against the wishes of their more radical members, they conclude that their best chance of securing the higher-ups’ attention lies with Henry Hunt (a superb Rory Kinnear), a celebrated orator of upper-class origin but democratic conviction; it’s around an address from him that a march and non-violent demonstration is planned in St. Peter’s Field in Manchester.

Such a potted synopsis does scant justice to the complex tapestry Leigh builds around this fateful gathering. At once exhaustive and exhausting, his screenplay also grants generous speaking time to such factions as the Manchester Female Reform Society and the journalistic staff of the left-wing Manchester Observer. As Hunt is built up by his followers as the man of the hour, “Peterloo” takes a longer view, its baggy structure and speechifying tone emphatically underlining the truth that it takes an entire community to bring about even the most incremental change.

That’s a mixed blessing: Carried by so many voices, the film is rhetorically relentless, but that leaves little air for the finer details and foibles of character that are the director’s stock-in-trade. His salt-of-the-earth subjects may be shot in exquisitely dank half-light by Dick Pope — whose compositions, as in “Mr. Turner,” deftly channel early 19th-century painting at a variety of scales — but exhibit little inner life beyond their stoicism. Only Hunt, wittily played by Kinnear as a trip-switch contradiction of noble righteousness and foppish entitlement, really emerges with multiple dimensions.

Yet when “Peterloo’s” unaligned fingers form a fist, for a punching, unyielding, robustly choreographed finale of rage against the right-wing machine, the film makes good on its most taxing demands. Edited with sharp, stabbing clarity by Jon Gregory, the massacre itself isn’t shot with the widescreen grandeur that war-film tradition has led us to expect; rather, Leigh keeps the focus tight on its pettiest acts of violation and violence, fury building with each body that falls. The atrocity at St. Peter’s Field isn’t presented as a climax, much less as a catharsis: It’s merely a concentrated demonstration of the unending social injustice that the film has spent the past two hours patiently tracking. “Peterloo” is, in some respects, an out-of-character work from Leigh: flawed, overcome, playing against some of his strengths while revealing other, unexpected ones. At its biggest moment, however, it’s the distinctive, stirring smallness of his filmmaking that pays off.