A woman says her night at the opera turned into a nightmare at the 20th Precinct after she misplaced her ticket during a production of “Hamlet” at the Met.

In papers filed in Manhattan Supreme Court, Aviva Aranovich, a classical pianist, says a hotheaded cop escorted her out of the storied opera house by smashing her head into a railing and letting her fall down a marble stairway — and then slammed the door of his patrol car on her ankle.

Aranovich’s Wagnerian hell didn’t end there — the officer then repeatedly smacked her on the head at the station house and denied her medical attention before two other cops took her to the hospital, where she was diagnosed with a concussion, her suit says.

The suit alleges “brutal police conduct” and “excessive force.”

Cops said the woman became “belligerent” when they showed up at Lincoln Center.

“She struggled with the officers” and hurt her ankle kicking out a window in the police car, said NYPD spokesman Paul Browne.

She refused EMS aid and was taken to Bellevue for psychiatric observation; her abuse claim was investigated and the cop was “exonerated,” he said.