A federal judge yesterday rejected an attempt by Michael V. Townley, convicted assassin of former Chilean ambassador Orlando Letelier, to win his release pending an effort by Argentina to extradite him for the 1974 car bombing of a senior Chilean exile and his wife in Buenos Aires.

U.S. District Judge Albert V. Bryan Jr. rebuffed arguments by Townley's lawyers that the 40-year-old ex-Chilean secret police agent, arrested as he sat in a prison cell about to be paroled, is being held in violation of both his constitutional rights and a plea agreement reached in the Letelier case.

Townley, now being held by U.S. marshals at Argentina's request, faces an extradition hearing scheduled for June 17 in federal magistrate's court in Alexandria. Argentina has said it will furnish evidence of Townley's complicity in the 1974 slaying by that time, Assistant U.S. Attorney Leonie Brinkema told Bryan yesterday.

Townley, who was granted parole last month after serving 62 months of a 10-year prison term for the 1976 Letelier slaying, was arrested on May 6 as he awaited his release from an undisclosed U.S. prison. Townley subsequently was held for several days stripped to his shorts in a padded cell under an unnecessary "suicide watch," his lawyer charged yesterday.

Townley's arrest has been linked in news accounts both here and in Argentina to the arrest on May 16 in Buenos Aires of Luis Arce Gomez, Bolivia's former interior minister and a suspected cocaine smuggling kingpin wanted by U.S. authorities in Miami. Argentine and U.S. officials have denied the timing of the two arrests is more than a coincidence, but a Justice Department spokesman has said it is "a cooperative law enforcement effort."

Townley, who testified twice for the government in trials here related to the car bombing in which Letelier and a coworker were killed as they drove on a Northwest Washington street, appeared in court yesterday in Alexandria under heavy guard. His lawyer, Jeffrey M. Johnson, said Townley served his prison sentence under an alias arranged by federal witness protection authorities.

Johnson argued yesterday that an arrest warrant for Townley signed by Bryan violated Townley's Fourth Amendment protection against illegal seizure because Argentina has failed to show there is probable cause to hold him. Johnson scoffed at U.S. efforts to hand over an American citizen to a regime that, he said, "detains and murders its own citizens at will."

Johnson maintained in an interview afterward that Townley "faces a severe threat to his life" if he is extradited to Argentina. "People routinely disappear there both in and out of prison," said Johnson. He said officials in power now in Chile have "a very strong motive to see that Mike doesn't breathe."

Prosecutor Brinkema contended that Argentina has committed itself to the State Department that it "is not acting in a random manner" and that "the necessary documents will arrive."

Bryan, noting that the allegations against Townley had been spelled out "with some particularity," ruled that Argentina had shown sufficient probable cause for holding Townley under terms of a 1972 extradition treaty between the two countries.

Townley has been charged in Argentina with taking part in the Sept. 30, 1974, bombing of Gen. Carlos Prats, a critic of Chilean President Augusto Pinochet, and Prats' wife as they parked their car at their Buenos Aires home.

A passport used by Townley under an alias in 1974 and introduced into evidence during the Letelier trials here showed that he was in Argentina until days before Prats' death. The passport was subsequently shown to Argentinian officials investigating the murder and is said by U.S. prosecutors to be a key element in the case against Townley.

According to a federal investigator, the Argentines also have a confession from a Chilean paramilitary leader who allegedly worked with Townley in the Prats killings.