A federal jury in Alexandria, after deliberating for more than 12 hours over two days, yesterday convicted former State Department official Walter Reed Martindale III of conspiring to transport firearms as part of an assassination plot against a flamboyant Saudi sheik.

The nine-man, three-woman jury found Martindale, of Fairfax County, guilty on seven of nine charges in the conspiracy, including impersonating a U.S. diplomat and transporting an Uzi semiautomatic rifle from Tampa, Fla., to Petersburg, Va.

Martindale, whose efforts to evacuate Vietnamese refugees after the fall of Saigon have been called "heroic," faces a possible 35 years in prison on the convictions.

In U.S. District Court in Alexandria, Judge Albert V. Bryan Jr. set sentencing for Nov. 2 and ordered Martindale, 41, taken into custody pending payment of a $25,000 bond. He had been free on a $10,000 personal bond since his indictment in July.

A resident of the Hollin Hills section of Fairfax County, south of Alexandria, Martindale stood with his hands clasped behind his back last night as the verdicts were read. Wearing a gray sports jacket and burgundy tie, he showed little emotion.

Afterward, one of his attorneys, Paul R. Kramer, said his client "was shocked at the verdict and didn't understand how he could be found guilty." Kramer explained Martindale's lack of emotion, saying, "He's been a diplomat so long, he's trained not to lose his composure." Until 1981, Martindale was an employe of the Agency for International Development.

The verdicts follow three days of testimony in a week-long trial with all the makings of a mystery novel, including allegations of the involvement of Cuban assassins and high Saudi officials and royalty, none of whom was ever fully identified.

Throughout the trial, prosecutors outlined a conspiracy in which Martindale obtained a gun, carried out surveillance and tried to recruit others in a plan to assassinate Mohammed Fassi, a member of the Saudi family, or members of his family, then living in London.

Martindale was arrested last October for taking an Uzi semiautomatic rifle and a .38 caliber revolver into London. As a result of that arrest and Martindale's plea of guilty in a London court to possessing the weapons, the U.S. government launched its own investigation.

Throughout his U.S. trial, Martindale's attorneys argued that the government's case was based on a series of misconceptions and misunderstandings, that Martindale had in fact been working on a legitimate business proposal involving security for rich Arabs.

Martindale testified that he had purchased the gun at the request of Ibrahim Rawaf, his partner in their Washington-based American International Trade Group. According to the prosecution, Martindale was to receive $50,0000 for his financially troubled company from unnamed Arabs for his part in the plot. Rawaf, who was also charged in the conspiracy, is a fugitive in Lebanon.

Assistant U.S. Attorney Justin Williams challenged the purported security proposal, noting that major evidence -- Martindale's compilation of snapshots of the Fassi family's London home and vehicles, and notes outlining the family's comings and goings -- were not the stuff of a multimillion-dollar business deal.

A federal indictment handed down in July charged Martindale with participating in the conspiracy. Martindale was found not guilty yesterday of two additional charges of transporting the Uzi through National Airport to Gatwick Airport in London.

Defense attorneys Kramer and Thomas Burch said they will file a motion for a new trial next week. "We still feel the evidence was not sufficient on some of the major charges," Kramer said.

Outside the courtroom, Martindale's mother broke into sobs, saying, "You know it isn't true . . . . I don't understand."

Martindale's parents, residents of Pensacola, Fla., sat calmly behind him each day throughout the trial. His father called the charges "trumped-up." "There was a conspiracy . . . and [Martindale] just got caught holding the bag," he said. He criticized the judge for his prompt incarceration of Martindale as "inhumane," noting that his bachelor son has "two little children," Vietnamese orphans he adopted.

Bryan ordered Martindale into custody after an appeal by prosecutor Williams, who noted that Martindale is "a man who's traveled all over the world." Williams also indicated concern that "there are other conspirators out there."