An accused Lebanese hijacker, both of whose wrists were broken when he was arrested by FBI agents at sea last week, was ordered held in preventive custody yesterday to await trial on charges of hostage-taking.

Fawaz Younis, 28, wearing casts from his palms to his elbows, listened glumly at a hearing in U.S. District Court here as government prosecutors played back a 1985 videotape of Younis at the Beirut airport, announcing apologetically that he and his colleagues were about to blow up the plane near where they stood.

Two U.S. citizens and two U.S. nationals had been passengers on the Royal Jordanian Boeing 727 jetliner, authorities said, giving the United States jurisdiction over the hijacking under an untested 1984 law aimed at international terrorism involving American victims. One of the four, William Slade, testified earlier yesterday before a federal grand jury, sources said.

U.S. Magistrate Jean F. Dwyer said at the hearing that the 1984 statute was "certainly open to attack" on a number of grounds, but added that this issue was not before her. Accordingly, she said, "I do not see that I have any alternative but to order the pretrial detention of the defendant."

FBI agent John F. Lipka, who signed the affidavit used to obtain a Sept. 11 arrest warrant for Younis, testified that "around 10, maybe less than 10" of the passengers who were interviewed identified Younis as "Nazeeh," the leader of the five heavily armed hijackers who stormed aboard the plane on the morning of June 11, 1985, as it was preparing to take off for a flight to Amman, Jordan.

The gunmen, identified at the time as Shiite Moslems, commandeered the plane for a fruitless two-day flight around the Mediterranean, seeking a conference with Arab League leaders in Tunisia only to be denied landing rights and finally returning to Beirut where they let off the passengers and crew before blowing up the plane.

A dark-haired man with a luxuriant beard, Younis was apparently set up by a close friend and lured onto an FBI-rented yacht in international waters off Cyprus on Sept. 13. The circumstances of his arrest were not discussed at yesterday's hearing, but sources said Younis evidently sustained hairline fractures of both wrists when FBI agents slammed him to the deck of the boat as they placed him under arrest.

Justice Department spokesman Pat Korten told reporters that Younis complained his wrists were "swollen" when he was transferred from the yacht to a U.S. naval vessel shortly after he was seized. Korten said there was "a lot of medical help" on the Navy ship in case of possible bloodshed, but "no X-ray" equipment.

Brought to Washington Sept. 17 following a nonstop flight from the aircraft carrier USS Saratoga, Younis was immediately arraigned in federal court here, pleaded not guilty, and then was given X-rays.

"It was found he had a small fracture in each wrist," Korten said. He said the department does not "have all the facts" because the arresting agents had yet to be interviewed. But a senior Justice Department official voiced confidence that the incident would be found to have been "accidental."

The 1985 videotape also showed Younis shaking hands with the passengers and kissing members of the flight crew as they deplaned, but FBI agent Lipka said "numerous stewards" and Jordanian skymarshals were beaten during the ordeal.

Younis' court-appointed lawyer, Frank Carter, protested at the hearing that no Americans were injured, that the hijacking was not aimed at Americans and that the airliner had apparently never flown over any U.S. territory or installation.

Justice Department lawyer Karen Morrisette said the 1984 statute, which carries a life penalty in Younis' case, does not require violence against Americans.

Morrisette also argued that Younis, said to be a full-time member of Lebanese Justice Minister Nabih Berri's Amal militia, has every reason to try to return to Lebanon "and find a safe haven there."