Sharp, swift, and lethally compact, “Los Olvidados” is a celluloid switchblade swiped at the jugular of city living. After more than half a century, Luis Bunuel’s expose of “the young and the damned” (as the movie was hyperbolically retitled in English) remains as stark and savage a movie as they come.

Starting today, Film Forum returns this ferocious masterpiece to the big screen in a new print with new subtitles. After each screening, an alternative “happy ending,” discovered in 1996, will be shown – an idea which is almost funny: Imagine a version of “Au Hasard Balthazar” where the donkey gets up and prances off into the sunset.

Shot quick and cheap on location in Mexico City, “Los Olvidados’s” highly original fusion of neorealism and surrealism tracks the hardscrabble existence of a gang of street children. Fresh out of jail, vicious Jaibo wastes no time beating his snitch to death, accosting a blind street musician, seducing a friend’s mother, and generally tearing the social fabric to shreds.

Like the dollar bill in “L’Argent” (this is the most Bressonian of Bunuel’s films), Jaibo circulates through, and corrupts the narrative. Everyone is subsumed by his negative tide. Our sympathies align with Pedro, a young boy whose attempts to transcend his circumstances are crushed by the overwhelming nihilism of his milieu. Bunuel’s depiction of shantytown Mexico rivals the postwar landscapes of de Sica or Rossellini for vivid immediacy, but only the historian of “L’Age d’Or” would dare spike it with such dark flourishes of imagination.

The movie is one bold image after another: a young girl pours milk over her thighs, triggering explosive lust in a voyeur; a legless pauper is lifted from his cart and kicked into the gutter; a ravenous boy suckles at a donkey’s teat; a murdered child is tossed in a garbage heap; a frightened dove is rubbed along the spine of a woman to cure her sickness; a wild dog haunts the mind of a dying adolescent.

There isn’t a single good shot in “Los Olividados.” They’re all perfect. His signature camera move is a dispassionate creep toward or away from the actors; the film examines its sensational subject with clinical cool, presenting us with a damning set of facts. Skeletal construction sites loom over the barren landscape like some nightmarish Sol LeWitt. In the famous dream sequence, a child’s guilty conscience is confronted by a slow-motion apparition in white, clutching a slab of raw meat – his mother. Clubs, knives, and insolent glares bristle at the camera; in one startling scene, an egg is thrown directly at the lens.

Bleak as it is, the material is ultimately less demoralizing than invigorating, and exhibits none of the cynicism, opportunism, or faux-indignation of a movie like “City of God” – one of dozens of movies indebted to this unsentimental masterpiece.

Until February 10 (209 W. Houston Street, between Sixth Avenue & Varick, 212-727-8110).