A military jury last night found Marine Sgt. Clayton J. Lonetree guilty of espionage.

The eight-officer jury deliberated 3 1/2 hours at Quantico Marine Base before convicting him of all 13 charges, including spying and failure to report contacts with Soviet nationals while guarding U.S. embassies in Moscow and Vienna.

Lonetree "shook" as he was pronounced guilty of all counts, but he "took it like a Marine," his defense attorney, William Kunstler, said after the court adjourned.

He was allowed 10 minutes with his mother, Sally Tsosie of Tuba City, Ariz., and aunt, Kathy Lonetree of Denver, before he emerged from the courthouse in handcuffs. He was driven back to the brig where he has spent the past eight months.

He showed no signs of emotion but grinned when his defense team applauded him as he was led to the vehicle.

His mother, holding an eagle feather in her hand, shouted, "Innocent!" as her son passed. She told reporters Lonetree "never expected a fair trial."

"I will appeal this," a visibly upset and angry Tsosie vowed. "He is not a traitor."

Kunstler confirmed that Lonetree would appeal the verdict, which marks the first time a Marine has been found guilty of espionage in the four decades they have guarded U.S. embassies.

Kunstler said his client is "the scapegoat for the State Department, the CIA, the Naval Investigation Services and the Marines."

"The trial judge {Navy Capt. Philip Roberts} violated his oath of office. He did not administer fairly," Kunstler said.

The court-martial board is to reconvene Monday to hear evidence before deliberating on a sentence. The jury is empowered to impose life imprisonment. The jury took the case after a day of final arguments by Marine prosecutors, who compared Lonetree to Benedict Arnold, and civilian defense attorneys, who depicted the 25-year-old enlisted man as a self-appointed free-lance double agent who wanted to expose or trap a KGB agent known as "Uncle Sasha."

Lonetree had pleaded not guilty to 13 counts of espionage, larceny and conspiracy.

Lonetree was motivated by "money, intrigue, sex, ego and ideology" to spy for the Soviets, the prosecutor, Maj. David L. Beck, said in his closing argument.

Kunstler, in his summation, portrayed Lonetree as a "foolish young man who thought he could do what . . . you cannot do: expose the KGB."

The prosecution case centered on sworn statements Lonetree made to Naval Investigative Service (NIS) interrogators shortly after he voluntarily told American officials in Vienna last December he had met several times with a Soviet KGB secret police agent.

In statements admitted into evidence during the eight days of testimony, Lonetree said he had given sensitive material to the Soviets between February and December of last year.

The defense called no witnesses, claiming to have been prevented from calling several by Judge Roberts. Outside court yesterday, Lonetree's lawyers said they did not call him as a witness because they thought he might "break down" under cross-examination.

Lonetree admitted having an illegal, unreported affair with a Soviet translator at the Moscow embassy, Violetta Sanni, from January to March 1986.

He referred to her as "my darling" in letters presented in court and was seen to weep when KGB expert John Barron testified that sexual entrapment was a common method of the KGB, which Barron said long had wanted to recruit a Marine guard as a spy.

Sanni introduced Lonetree to a Soviet known as Alexei (Uncle Sasha) Yefimov, described by prosecutor Beck as the Marine's KGB case officer. The court was told that American diplomats in Moscow had long regarded Yefimov as a "very sharp" source of information about Soviet leaders. The diplomats apparently knew nothing of Yefimov's clandestine ties to Lonetree.

Lonetree gave Sasha blueprints of certain floors of the Moscow embassy, identified Central Intelligence Agency agents at the Vienna embassy and received $3,500 from the Soviets.

Lonetree revealed his clandestine activities to a man understood to be CIA station chief in Vienna, identified only as "Big John" in court. There followed 11 days of interrogation by CIA and NIS agents in Vienna and London, after which Lonetree signed the statements that formed the core of the case against him.

"If he had not come forward, Yefimov might be out there yet with . . . other Marines in Moscow. It took enormous courage to do that . . . he voluntarily put himself in jeopardy," Kunstler argued.

Beck said the defense had "mistaken the fictional character of Walter Mitty for the real-life character of Benedict Arnold . . . . While the accused wore the uniform of a United States Marine, he betrayed his country."

The Marine held beliefs "antithetical to the American way of life," Beck said. Beck said Lonetree had "never outgrown" adolescent opinions of anti-Semitism and admiration for Adolf Hitler, and introduced a high school notebook of the Marine's with a swastika drawn on the cover.

The court heard how Lonetree, an American Indian from St. Paul, Minn., had told fellow Marines before going to Moscow that he "wanted to be a KGB spy" and would leave the Marines if he did not get the Moscow posting.

Once there, Lonetree had several ideological arguments with fellow Marines, one of whom, former guard J.R. Muldowney, told the court Lonetree had on several occasions defended communism and fascism.

One NIS agent testified he believed the defendant's motivation included "hatred for the white man," while another testified that Lonetree told him Sasha had asked Lonetree to bug the ambassador's office in Vienna and identify any fellow Marines who might be susceptible to KGB pressure. Lonetree had discussed another Marine, but had refused to plant the bug.