Armed men reported to be Palestinians seized an Italian cruise ship with more than 450 persons aboard off the Egyptian coast today and demanded the release of about 50 Palestinian prisoners they said were being held in Israel.

While details of the commandeering of the vessel remained sketchy, the captain of the ship, Tristone Benni, was able to tell the Port Said port authority by radio late tonight that the hijackers were members of a group called the Palestine Liberation Front and that they threatened to kill passengers if the 50 Palestinian prisoners are not released, according to diplomatic sources.

As of late Monday night, there was no statement by the hijackers or by any Palestinian groups, although diplomatic sources in Cairo said the gunmen threatened to blow up the ship if their demands are not met or if any other boat approaches.

The circumstances of the hijacking were still unclear as the liner Achille Lauro drifted 30 miles out from Port Said in the Mediterranean.

No official numbers of passengers or breakdown by nationalities were available, but an Italian Foreign Ministry source in Rome said there were 28 Americans among the passengers on the ship, Washington Post correspondent Loren Jenkins reported.

According to tour directors and passengers here, about 135 among the ship's passengers were Americans. Of these about 70 were among a group of more than 600 passengers who left the boat in Alexandria for an excursion to Cairo. The group was to reboard the ship in Port Said.

In the lobbies of the hotels where many of those passengers were taken, there were scenes of desperation and shock. One Italian woman had left her two children, aged 4 and 5, on board while she had come to see the pyramids. Harriet Hauser, of Hollywood, Fla., said she was traveling with a group of four older women. Two of them stayed on board, she said, because one of them had a heart condition.

The Italian Foreign Ministry source said that according to radio reports from the ship relayed by a ham radio operator in Sweden, the gunmen had locked passengers in their cabins and were systematically checking identity, nationality and religion of all aboard.

Italian Foreign Minister Giulio Andreotti reportedly contacted Palestine Liberation Organization Chairman Yasser Arafat upon learning of the Achille Lauro's seizure, and the PLO leader said his people had nothing to do with the incident. Italian sources said Andreotti pressed Arafat to help resolve the situation. Andreotti reportedly was also in touch with Egyptian officials.

Spokesmen for the PLO in Cairo and at the United Nations in New York said they had no knowledge of the incident and declined to comment further.

The incident comes less than a week after Israeli jets bombed the PLO headquarters in Tunisia in a devastating long-range attack retaliating for the slaying of three Israelis on a yacht in Cyprus.

The PLO has attempted to mount at least two seaborne operations during the past few months off the Israeli and Lebanese coasts.

The Achille Lauro had been carrying 780 passengers and 350 crew, according to Italian diplomats. Most of those still aboard appeared to be crew. The Israeli Army radio reported tonight that there were apparently no Israelis aboard the ship.

The Achille Lauro normally travels from Genoa to Naples, stops in Greece, Alexandria, Port Said and Haifa, Israel, travel agents here said.

There was no clear indication of whether the hijackers boarded the ship at one of the stops or landed from another vessel.

In another development, the Israeli Army command said Monday night that a security patrol intercepted a band of Palestinian guerrillas and killed four persons responsible for a chain of killings of Israelis in the Judean Hills of the occupied West Bank during the past year.

[The Army command said that the band had killed five Israelis since May 1984 and wounded at least 18 others.]