In the first public appearance here by two Libyans accused of bombing Pan Am Flight 103, a prosecutor stumbled today trying to stop a defense motion to throw out a key charge against them.

The pretrial hearing was an early sign of the intricacy of the case and the unprecedented nature of the proceedings, which are being conducted by a Scottish court in the Netherlands.

The two defendants, allegedly agents for Libyan intelligence, were turned over by Libya in April after a unique agreement with the United States and Britain to hold the trial in a neutral country.

Prosecutors charge Abdel Basset Ali Megrahi and Lamen Khalifa Fhimah carried out the Dec. 21, 1988, bombing over Lockerbie, Scotland, that killed all 259 people aboard the plane--including 189 Americans--and 11 people on the ground.

The case against the two could be hurt, however, if the defense succeeds in getting charges of conspiracy to murder thrown out. That would make murder charges harder to prove, legal experts said.

When the judge pressed prosecutor Colin Boyd on why the Scottish court should have jurisdiction to try a conspiracy that allegedly was put together elsewhere in Europe and North Africa, Boyd appeared flustered and confused. After a long silence, he effectively admitted that his only basis was that the final crime--the attack on the jet--occurred in Scotland, an insufficient basis for jurisdiction under Scottish law.

"Without the overt act it is doubtful that anyone could prove the conspiracy," Boyd conceded.

Lord Ranald Sutherland, a Scottish high court justice, adjourned the hearing until Wednesday when, he said, he "may or may not" issue a ruling on the defense motion. If his ruling is postponed or appealed by either side, it could delay the start of the trial.

Outside this former U.S. air base, Scottish police armed with semiautomatic weapons were on patrol. Inside, prosecutors and lawyers wearing white wigs and robes argued the case in a converted sports arena with rain audible on the metal roof. The trial is scheduled to begin Feb. 2.

The defendants, wearing suits and ties and seated behind a bulletproof shield, followed the proceedings intently with the aid of Arabic interpreters.

CAPTION: British artist Elizabeth Cook sketches the Libyan defendants during a pretrial hearing in the Pan Am bombing case in the Netherlands.