A U.S. District Court judge has ruled that when FBI agents and Alexandria policemen broke into the apartment of a woman they erroneously believed was newspaper heiress Patricia Hearst, they violated the woman's constitutional rights.

U.S. District Court Judge Robert R. Merhige Jr. ruled that when the agents forced their way, guns drawn, into Elizabeth Ann Norton's Alexandria apartment on March 15, 1975, they violated the Fourth Amendment, which protects a person from unreasonable searches and seizures.

The ruling, handed hown in Richmond this week, did not deal with the question of damages. John D. Grad, Norton's attorney, said he expects a date for the damage hearing to be set soon.

Norton, who now lives in West Virginia, had sued the FBI and the city police department for $500,000. At that time of the raid, Hearst was the object of a nationwide search.

Grad and Assistant U.S. Attorney James R. Hubbard said yesterday they have not yet received a copy of the judge's memorandum, but Grad said he has been informed of its contents.

Grad said the judge's ruling means that the government can "be held liable for the wrongful acts of its agents, whether or not they acted in good faith."

FBI agents and Alexandria policemen forced their way into Norton's apartment after receiving an anonyments identified the Norton apartment as the hiding place for Hearts. At the time of the break-in, FBI agents said they were looking for Norton's roommate, who supposedly resembled Hearst.

Last August, the government filed a brief with the court saying that the agents and policemen who raided the apartment demonstrated "care, caution and clear respect for her dignity." They asked the court to dismiss the suit and not award any damages.