FRANKFURT, West Germany (AP) _ Confessed TWA hijacker Mohammed Ali Hamadi today contested evidence against him, claiming a taped conversation between Beirut Airport authorities and the seized plane were from another hijacking.

Hamadi, a Lebanese Shiite Moslem accused of murder and air piracy in the June 1985 hijacking, interrupted testimony to say the Arabic-language conversation introduced as evidence was not an exchange between him and the Beirut control tower.

''I think I recognize now that the last part of the tape concerns another hijacking that had nothing to do with TWA,'' Hamadi told the court. He did not say what hijacking he believed the tape referred to.

Alen Mousa, the Lebanese who translated the taped conversation for West German authorities, was testifying when Hamadi interrupted him. Mousa told the court he could not be sure if any of the voices on the tape belonged to Hamadi.

Hamadi's defense attorneys made a motion for further examination of the tape.

During the 17-day hijacking ordeal, U.S. Navy diver Richard Stethem was killed and 39 Americans were held hostage aboard TWA Flight 847, seized en route from Athens to Rome and diverted to Beirut and Algiers.

Hamadi has admitted he and an accomplice hijacked the plane, but he denied killing Stethem. His alleged accomplice, Hassan Ezzeddine, remains at large.

On Tuesday, Houston businessman Allyn Conwell told the court he was so ''terrorized that I was virtually immobile'' while on board the seized plane. He said he feared the captors planned to shoot him and other Americans who were taken to hiding places in Beirut.

Four American servicemen were removed from the plane by Hezbollah, a radical Shiite Moslem group backed by Iran, while Conwell's group was in the custody of Amal, a Shiite militia that collaborated with the hijackers.

Conwell said he learned from members of Amal, whom he described as less extreme than the two hijackers, that Hamadi and his accomplice wanted to bring all of the hostages back on board and blow up the plane.

''I was under the impression that Amal absolutely did not approve of the hijacking and the murder of Stethem,'' Conwell testified. ''I think they acted to the best of their ability in a chaotic situation.''

Conwell said members of Amal told him ''the original hijackers were getting very upset over the lack of progress in negotiations'' for the release of the hostages.

''I was told the intention was to return all the hostages to the plane and blow it up,'' Conwell said.

He said Amal officials indicated to him that the two hijackers were from Hezbollah. Hamadi has denied being a member of that group.

Also Tuesday, another former American hostage testified that Hamadi, when told an American serviceman aboard the plane was suffering chest pains, responded ''Let him die.''

Sue Ellen Herzberg also told the court that she felt ''degraded to an animal state'' during the hijacking.

Hamadi, who was arrested at Frankfurt Airport in January 1987 after customs officials found liquid explosives in his luggage, has been on trial since July.