Four Libyan diplomats whom the United States ordered to leave the country by Monday have holed up inside their country's mission here, and State Department officials yesterday all but conceded that they do not know what to do about it.

The four were ordered expelled for their alleged role in a mounting campaign abroad of intimidation against opponents of the government of Muammar Qaddafi. Speeches by Qaddafi and other official Libyan pronouncements since February have called on Libyan dissidents to return home or face "liquidation."

Four Libyans have been killed in Europe since mid-March, and officials believe their deaths are linked to the Libyan intimidation campaign.

On Friday, Nuri Ahmed Swedan, Ali Ramram, Mohammed Gamudi and Abdulla Zbedi were told that they had 72 hours to leave Washington, and the United States recalled its last two diplomats from Tripoli. Last month, two other Libyan diplomats were ordered to leave for similar reasons, but they compiled.

The spokesman for the Libyan mission, which calls itself a "people's bureau," yesterday challenged U.S. authorities to prove allegations that the four were involved in any illegal activity.

"They will not depart the United States voluntarily," the spokesman, Ali Houderi, said. He said they were prepared to remain inside the mission indefinitely.

Meanwhile, uniformed police and federal agents waited outside with orders to take the four in custody when they emerged and escort them to the airport.

"As of today, we have to see what happens," a State Department official said yesterday, adding that the United States was considering what steps it might take to force the defiant diplomats to leave. Officials called the Libyans' refusal to obey the U.S. order "unprecedented" and said only that American authorities would not violate the physical premises of the Libyan mission.

The four diplomats who were ordered to leave from, with Houseri, the five-member "people's committee" that has run the people's bureau since Libyan students took over their embassy, at Qaddafi's instigation, here last September.

Libyan exile sources say they fear that "hit teams" dispatched by Qaddafi could strike next in the United States, and a State Department official echoed this concern yesterday.

"We hope, frankly," he said, however, "that the steps we have taken will help deter the campaign of intimidation."

U.S. officials say the worldwide intimidation campaign apparently is linked to internal unrest and a major purge of dissidents inside Libya.

Authorities in several European countries -- particularly Britain and Italy, where the four recent slayings took place -- have been on the alert to prevent possible attacks against Libyan exiles, and the FBI and U.S. immigration official are investigating possible illegal activities by progovernment Libyan students.

Libyan students in the United States have received literature from their government, much of it -- according to documents made available to The Washington Post -- sent by the Libyan mission, outlining the Tripoli government's plans to carry out the "physical elimination of the enemies of the revolution abroad."

An Arabic-language newspaper in London, Al Shark Al Jadid, recently published what it described as "death lists" obtained from Libyan intelligence sources including 20 names of Libyans in the United States.

The Libyan mission here has sent notices to Libyan students to alter their visa status to one that would make it more difficult to extend their stay in the United States and has ordered all Libyan students not on Libyan government scholarship to return to Libya or "face the consequences."

The mission also has solicited names of Libyans studying here from U.S. colleges and universities in what Qaddafi's opponents see as a ploy to control their activities.

In the fight to resist the U.S. expulsion order, the Libyans have pointed out that the four do not have diplomatic visas and have student status. This would prevent them from expulsion as diplomats, since they technically are not diplomats, they said.

The State Department maintains, however, that the mission has been accepted as a diplomatic establishment in accordance with the Vienna Convention governing diplomatic relations, and that the Libyans have accepted this status.

"The question is not what kind of visas they have, but in what capacity are they here," a State Department official said.

Libya is the source of 10 percent of U.S. oil imports and is the third-largest supplier of foreign oil to this country. Diplomatic relations between the two countries have been strained, however, since a Libyan mob stormed the U.S. Embassy last December.

U.S. officials said yesterday that there are about 2,000 American business representatives in Libya and that the State Department is keeping them informed of steps taken against the Libyans here. Americans were warned in December to minimize travel to Libya.

Meanwhile, a Libyan broadcast last night said a message had been sent to President Carter asking him to "end the siege by the American police forces" of the Libyan mission.