Radical Iranian students holding 50 Americans hostage at the U.S. Embassy here produced documents tonights that they said proved an embassy political officer was a spy, not a diplomat.

The documents included a forged Belgian passport, a set of instructions detailing the cover to be used and a set of immigration stamps to make it appear as if the passport had been used to enter or leave Iran through Mehrabad airport here. The students said the forged passport, belongs to Thomas Leo Ahern Jr., 47, whom they identified as a political officer and narcotics control coordinator for the embassy.

The main justification the Iranians have made for holding embassy personnel hostage is that many were spies. There have been persistent threats to put the hostages on trail for spying, and demonstrations such as today's were staged in an effort to buttress those claims.

"Ahern was listed as a political officer but we will prove he is a spy," said a spokeman for the students. "We don't accept him as a diplomat," indicating that the American may face a trial as a spy.

[In Washington, the State Department refused comment on the charges.]

Ahern is the third American in the embassy said by the students to be a spy. The other two, Malcoln Klap and William Daugherty, allegedly were identified as a result of a secret cable saying they would be using a cover as diplomats to perform CIA activities.

It is unclear from the press conference today just why Ahern would be required to use the forged Belgian passport. It was also unclear as to whether he was in fact a CIA agent or used the passport, as part of his duties in narcotics control efforts.

The students exhibited the Belgian passport, which was made out in the name of Paul Timmermans, and Ahern's American diplomatic passport. They both bore identical photographs.

According to a document stamped secret that the students also exhibited, Ahern's cover was as a commercial business representative.

"According to personal data in your passport, you are single, you were born in Antwerp, Belgium, July 8, 1934, have blue eyes, no distinguishing characteristics and you are approximately 1.88 meters [just over six feet] tall," the secret cover instructions said.

It also discussed his languages, where he would live, and his job -- "traveling on business in the past as reflected in your passport, and now assigned to the Middle East section of your company." The passport was issued in the Belgan city of Jelle on March 3, 1977, reinstated on March 15, 1978, and set to expire March 14, 1982. It contains stamps indicating that Timmermans had visited Madrid, Lisbon, New Delhi, Helsinki and Athens.