Rampart, Oren Moverman, 102 Mins (15)

He's violent, mouthy, seductive, and an all-round scuzzball. Woody Harrelson is a man out of time in the cleaned-up LAPD. Still, he can always retrain and become a lawyer

Abel Ferrara made a film in 1992 called Bad Lieutenant, about an unhinged cop. Twenty years later, that seems an oddly redundant title. I mean, can you think of the last time you saw an uncomplicatedly heroic law officer on screen? In these disabused times, who would pay to see a film called "Ethical Detective"?

In literature, the authority on dirty, lowdown, scuzzball policing is American novelist James Ellroy, whose cops are variously corrupt, psychopathic, misogynistic, racist, homophobic and crypto-fascist, quite often all at once. On Rampart, Ellroy shares script credits with director Oren Moverman (following his debut The Messenger).

Unlike the period pieces Ellroy is known for (most memorably adapted in L.A. Confidential), Rampart is a near-contemporary story. It's set in Los Angeles in 1999, and the title alludes to a real-life scandal that blew up around then in a police division of that name in the Downtown area, with some 70 officers found guilty of corruption and brutality. Those last two words could be the middle names of Officer Dave Brown (Woody Harrelson) – although he's got just as juicy a sobriquet in "Date Rape Dave", after his killing of an alleged offender.

Brown establishes his personality in the opening scene – a conversation with two other officers in which, thanks to Moverman's love of overlapping dialogue and a very harsh sound mix, I could barely make out what was being discussed, just enough to gather that Brown is a prize creep. For the fun of it, he bullies a young officer over her French fries, then drives his patrol car (a free operator, he's always seen driving alone) straight into a huddle of Latinos.

Brown is forever getting himself into deep, murky trouble, and people keep asking him how it is that he's still on the payroll. He seems to represent the force's festering id – he's like Rampart's rotten 70 all rolled into one. (The film, by the way, isn't really about Rampart Division or its staff; rather, Dave Brown is himself a rampart, a walking bastion of badness).

What's most Ellroyesque about Brown is his unflagging verbosity. Hauled up before superiors and investigators (embodied variously by Steve Buscemi, Ice Cube and, at her most imposingly chilly, Sigourney Weaver), Brown coolly wheels out words like "hyperbolised" and "vicissitudes" or cites legal precedent. ("You have all these cases memorised?" asks a colleague; he replies, "If I don't, I make 'em up.") He's alarmingly knowing about his prejudices, and his intelligence; he tells a law firm that if he's drummed out of the force, "I'll study for the bar, come back and work as your token fascist."

Whether he's verbally fencing, beating suspects or picking up women in bars (at which he's horribly adept), Harrelson's hardcore anti-hero is mesmerising. Emaciated, as if boiled down to the barest physical specifications required for male toughness, the shaven-headed Harrelson looks grimly priapic, like a phallus whittled out of wood, topped by the face of a smirking gremlin. It's a jewel of a part for Harrelson, who's constantly on screen, strutting, simmering or letting rip in Brown's more excessive moments. These include raging at attorney and sometime bedmate Linda (Robin Wright) from her swimming pool in the rain, or blowing off steam in a thunderously polysexual nightspot – not as you might expect but by stuffing himself with tacos, then copiously spewing. (Director Moverman similarly lets his hair down here, hitting us with red light, strobe-paced editing and blasting electro-noise).

Brown's vulnerable side emerges in an unusual home life with two hippie-ish ex-wives (Cynthia Nixon, Anne Heche) who are also sisters. He has two daughters with them: the older, Helen (Brie Larson), a teenage rebel whose chilly contempt masks her wounded love. There's a superb long-take stand-off between Brown and his daughters in a hotel room, when he earnestly assures them: "I never hurt any good people." "What about us?" Helen retorts.

There's lots to relish in Rampart, including some juicy touches of Ellroy grandiloquence in the script: on TV, Brown is accused of "old-school LAPD brutality revisited, revived and regurgitated". So I'm not sure why the film falls somewhat short of brilliance. Rampart is a compelling, classy character study that's crammed with good qualities – almost too crammed. I can't remember when I last saw a film that wasn't an ensemble piece but was so full of notable cameos.

But the result is that you never quite get a grip on Brown's world or the people in it. Because this is a police story, you expect more of a cogent narrative – which doesn't mean that there ought to be one, simply that you expect one. Instead, Moverman imparts an oddly stoned sense of the world as its anti-hero drifts through it, a world that gradually shrinks and gets more claustrophobically rank as everyone in it loses their patience with him. One scene especially, an audaciously underplayed encounter between Brown and his sort-of mentor (that veteran in unpleasantness Ned Beatty) recaptures some of the bleakness of the great American paranoid thrillers of the Seventies (Night Moves, The Long Goodbye). Apart from occasional high-intensity moments, Rampart is surprisingly downbeat – like a leisurely gondola ride through a moral sewer. You can take that as a recommendation, mostly.

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