A Northern Virginia man accused of joining al Qaeda was captivated by the prospect of shooting President Bush or blowing him up with a car bomb because he wanted to kill "the leader of the infidels," prosecutors told a federal jury in Alexandria yesterday.

As the trial of Ahmed Omar Abu Ali began, Assistant U.S. Attorney David Laufman said the Falls Church man told his Saudi jailers that he was "determined" to kill Bush as part of an al Qaeda plot. The plan involved using three snipers firing simultaneously or a "martyrdom operation when Bush comes out to greet the people," Laufman quoted Abu Ali as saying.

"Ladies and gentlemen, the defendant in this case is charged with extremely serious offenses,'' Laufman told the 12-member jury in his opening statement. "He betrayed his country by joining forces overseas with our most lethal enemy, al Qaeda."

Defense attorneys focused on the torture allegations that have long hung over the case, one of the highest-profile terrorism prosecutions brought since the Sept. 11, 2001, attacks. They said Abu Ali, 24, lied to his Saudi jailers because he was whipped, kicked and beaten while a prisoner in Saudi Arabia until earlier this year.

"This is a case of an American student who was threatened, beaten and tortured into giving false confessions that weren't true . . . at the hands of one of the most brutal police agencies," Carmen Vizcaino said.

To dramatize Abu Ali's allegations that the Saudis chained him to the floor and whipped him on his back while screaming at him to confess, Vizcaino clapped her hands repeatedly and cried out: "One lash! Another! Another! Another! Another! Confess! Confess!"

A federal judge has already cast doubt on the torture allegations, ruling last week that Abu Ali's contention that he was whipped so hard his back was bloody seemed implausible because "all of the evidence" indicates that he was not in pain a few days later. U.S. District Judge Gerald Bruce Lee's opinion came after a six-day court hearing during which defense attorneys asked him to throw out Abu Ali's statements to the Saudis because, they said, they were made under duress.

Lee refused and allowed the statements into evidence. He said that defense attorneys may raise the torture arguments again at trial.

Abu Ali is charged with conspiracy to assassinate Bush as well as other terrorism counts in connection with the alleged al Qaeda plot, which prosecutors say envisioned a Sept. 11-style attack in the United States. Laufman told jurors yesterday that Abu Ali and members of an al Qaeda cell he joined in Saudi Arabia discussed crashing planes into targets in the United States, killing members of Congress and blowing up U.S. military aircraft.

The trial, in U.S. District Court in Alexandria, is expected to last three weeks. If convicted, Abu Ali faces up to life in prison. He was arrested in Saudi Arabia in June 2003 and held there before being flown back to the United States in February.

Among the government's first witnesses yesterday was a Saudi security officer, who testified via videotape from an earlier deposition he gave in Arabic in Saudi Arabia. For security reasons, only the attorneys, defendant and jury were allowed to view the testimony on television screens. An English translation could be heard in the rest of the courtroom.

The officer said he was one of seven Saudi security officers who arrested Abu Ali while he was taking final exams at a Saudi university and then searched his dorm room. The officer is expected to resume testifying today.

Ahmed Omar Abu Ali is accused of plotting to kill the president.