If Michael Mann made a movie in Iran it might look like this: a ferocious drama-thriller in which a haunted, morally ambiguous cop faces off with a despairing drug lord. We begin with a barnstorming chase sequence in which an officer runs after a drug dealer holding a bag of heroin; the scene climaxes with a shockingly nasty end for the dealer, setting a gruesome tone for the rest of the film.

The director is Saeed Roustayi, whose Leila’s Brothers was in competition at Cannes last year; this is in fact his previous film. Payman Maadi (from Asghar Farhadi’s A Separation) is police officer Samad, who is losing the “war on drugs”; gangsters have murdered the son of his subordinate, so his team’s dedication to bringing down the drug dealers has a new fanatical energy. Navid Mohammadzadeh is Naser, a drug kingpin who is already feeling his dominion crumbling.

Starting with a small-time bust, and bending the law with menaces and threats applied to the foot soldiers from whom he demands the names of those higher up, Samad and his crew ascend the food chain of evil to discover Naser in his penthouse, reclining in his hot tub in grisly circumstances. Once taken in, desperate Naser offers Samad frantically huge bribes; the officer’s response is strange. Is he tempted? Or does he simply, in his simmering contempt, want to make Naser cringe, to hold out the hope of release only to take it away?

The station house is an amoral inferno in which prisoners are crowded in a hellhole holding tank, like something by Héctor Babenco; cops can themselves be impugned and briefly handcuffed on the basis of culprits’ cynical claims of corruption and complicity, while a climactic execution scene is truly stomach-turning – a ritual slaughter which gives catharsis to no one. It is a grim and grisly world in which the “law of Tehran” feels like a futile rearguard action against chaos.

• Law of Tehran is released on 31 March in cinemas and on digital platforms.