An American-owned cargo jet being used by the Palestine Liberation Organization to transport 50 tons of arms to Nicaraguan rebels was temporarily detained in Tunisia and released last night after its cargo was unloaded, well-informed sources said.

The Boeing 707 owned by a Kansas City-based company, Global International Airlines, was chartered by the PLO through a Belgian company, Young's Cargo Service of Brussels, the sources said.

While the plane's cargo manifest showed that it was carrying medical and relief supplies, the craft actually carried arms and ammunition, including three artillery pieces. All the arms reportedly were of Chinese origin.

It was the first known instance of PLO weapons shipments to the Sandinista had received PLO training in Algeria a few years ago.

In February, the PLO issued a joint statement with a part of the Sandinista leadership condemning Israel.Israel has supported Nicaraguan President Anastasio Somoza and has sold generous amounts of arms to his National Guard.

The Somoza family's support of Israel goes back to 1948 when Nicaragua supplied arms and passports to Jews seeking to establish their state in Palestine.

The Tunisian government last night issued a statement saying that "a chartered 707 belonging to a small U.S. company and under contract for the use of the Palestine Liberation Organization landed in Tunisia to pick up military equipment of non-Western origin belonging to the PLO."

The statement said that "the decision was taken by the government of Tunisia to unload the plane and return it to the care of the crew for departure" from Tunis to Amsterdam.

The plan took off from Tunis early this morning without the cargo but with the crew, American officials said.

Global International Airlines, in which several wealthy Iranians reportedly have principal interest, was established about a year ago and owns two aircraft, according to a company spokesman.

The spokesman, Stan Adler, said in a telephone interview that the Boeing was hired to carry Red Crescent supplies from Beirut to Costa Rica, which the Sandinistas use as a base for their struggle against Somoza.

Three Palestinians and a representative of Young's Cargo Service boarded the aircraft in Beirut after it was loaded with crates bearing Red Crescent symbols. The Red Crescent is equivalent of the Red Cross in the Moslem world.

As the craft was approaching the Tunis-Carthage airport, Capt. Paul Marable of Kansas City, Mo., was ordered to land at a nearby military field at Bizerte, about 40 miles from Tunis, according to Adler.

Marable and his three crew members, all Americans, were not present while the aircraft was loaded with about 30 additional tons of crates and three pieces of artillery. Adler said that when the captains saw the artillery, he refused to fly the aircraft to Costa Rica.

Other sources said the pilot told officials that the craft had some mechanical difficulties and would have to make a stopover at the civilian airport in Tunis. Once he landed in Tunis, the pilot called the U.S. Embassy seeking help.

State Department officials said that the crew held lengthy discussions with U.S. Ambassador Stephen Bosworth, but these officials would not disclose any details surrounding the three-day delay of the aircraft by Tunisian authorities.

Global officials said that the crew was held incommunicado for 24 hours at Bizerta before they were allowed to phone the U.S. Embassy in Tunis and company officials in Kansas City. The crew, all from the Kansas City area, reported they were treated well.

Apart from the Tunisian government statement, there were no official Tunisian explanation of the mysterious incident. The presence of PLO weapons at a Tunisian military airfield also remains a mystery.

Mohamed Gherib, Tunisian charge d'affairs here, said he had no information about the incident. He said that he was in contact with State Department officials who were involved here and in Tunis in protracted negotiations to have the plane unloaded and allowed to fly out of Tunisia.

After Tunisia announced last night that the plane was unloaded and would be allowed to fly to Amsterdam, a State Department spokesman said that "legal implications" involved in the incident were being studied.

The two top Global officials, chairman Farhad Azima and executive vice-president Mansour Rasnavad, said that the plane was diverted to Bizerta to be reloaded with arms without their knowledge or approval.