QUANTICO, Va. (AP) _ The first day of the sex-for-secrets court-martial of a former Moscow Embassy guard ended Wednesday with a defense lawyer saying he will fight prosecution plans to have an unidentified witness testify.

William Kunstler, who is heading the defense of Clayton Lonetree, said he would challenge a motion to the Military Court of Appeals allowing prosecutors to use the witness.

''That has never happened in the history of the country,'' Kunstler said after the court-martial adjourned for the day. Asked if the unidentified witness was a CIA agent, Kunstler said, ''Who knows?''

Kunstler has said the CIA tried to persuade Lonetree, a Marine sergeant, that he was being recruited as a double agent and deceived him into talking to them during an investigation of embassy security.

Most of the opening day's proceedings, including arguments about the witness, were conducted behind closed doors because they involved classified material. The court-martial resumes Thursday.

The military trial began with Kunstler asking about possible prejudice against their client.

''I think the question of him being an American Indian is a very important one,'' said William Kunstler, who is heading the defense.

Lonetree, 25, of St. Paul, Minn., is the first Marine ever tried on espionage charges.

He faces 13 counts, including espionage, involving alleged disclosure of classified embassy information such as floor plans and office assignments.

Kunstler said the information involved was ''pure junk, like telephone books'' and should not be classified. He also said prosecutors ''want to make a mountain out of a molehill'' of Lonetree's love affair with a Soviet woman.

Lonetree was escorted by five guards into the Quantico Marine Corps Base headquarters for the court-martial.

When the proceedings began, defense lawyer Michael Stuhff questioned the presiding judge, Navy Capt. Philip F. Roberts, on his feelings about American Indians. Roberts is a native of South Dakota, which has a large Indian population and several major reservations.

Roberts told the defense there was no concentration of Indians in the area of the state where he lived, and assured Stuhff he is not biased. He also said he was not biased against Kunstler, who has worked on such cases as the Chicago Seven trial.

Lonetree asked for a jury trial, but he did not invoke his right to have enlisted personnel appointed to the panel.

Most of the opening session was spent arguing motions, including whether certain testimony should be closed.

Kunstler and Stuhff said they wanted as much of the case as possible opened to the public. They said military prosecutors, in trying to close the court- martial, were trying to give the appearance that national security had been compromised.

Kunstler also said he wanted his client released during the trial and would ask for a delay to challenge a statement made by Lonetree to the Naval Investigative Service.

''There is no case without the NIS statement,'' he said.

Kunstler has said NIS agents initially failed to adequately warn Lonetree of his rights to an attorney, interrogated him at length without an attorney present and pressured him to fabricate statements about his Moscow activities.

If convicted of espionage, Lonetree could be sentenced to life imprisonment.

Lonetree's father, mother, grandmother and aunt - each carrying a single feather as a symbol of peace - attended the trial. His aunt translated the proceedings for his grandmother, who speaks only Navajo.

About a dozen American Indians staged a show of support for Lonetree outside the main gate of the northern Virginia base, 20 miles south of Washington, D.C.

Lonetree's father, Spencer, called his son, ''a victim of being a human being.''

''He fell in love with a woman. ... I have seen a picture of her. She's a very attractive woman,'' the elder Lonetree said.

He said his son was in good spirits and believed he would be cleared.

''He's a very dedicated Marine,'' the father said. 'He doesn't blame the Marine Corps.''

Lonetree has been held in solitary confinement for eight months awaiting trial. He was charged with providing Soviet agents with classified embassy documents, identifying American intelligence officers assigned to the Moscow embassy, and fraternizing with a Soviet woman.

The most serious charges, that he allowed Soviet agents inside the embassy, were dropped May 15 after Cpl. Arnold Bracy, another former embassy guard, recanted a statement implicating Lonetree.

Lonetree was the first Marine arrested in the embassy espionage scandal.

A Marine assigned to the Moscow embassy at the same time as Lonetree, Sgt. Kenneth J. Kelliher, 2, is awaiting a military judge's ruling on whether he will be court-martialed on charges of involvement with Soviet women, dealing on the black market and copying classified documents.

Staff Sgt. Robert S. Stufflebeam, 25, faces charges of failing to report his involvement with Soviet women. Charges against Bracy, 21, were dropped for lack of evidence.