The government of Prime Minister Mehdi Bazargan resigned today, formally turning over power to Iran's Moslem clergy as demonstrators occupying the U.S. Embassy threatened to kill their American hostages if the United States tried to free them by force.

The occupiers, whose action was condoned if not organized by the ruling Moslem clergy under Ayatollah Ruhollah Khomeini, were joined by rifle-brandishing reinforcements on the third day of their takeover.

At the United Nations, a Palestine Liberation Organization spokesman announced that PLO leader Yasser Arafat is sending a delegation to Tehran Wednesday to "secure the safety of Americans and others who are at the U.S. Embassy in Tehran."

In the meantime, PLO deputy U.N. observer Hassan Abdel Rahman said, Arafat has instructed PLO representatives in Tehran to use "all possible means to secure the lives of the hostages."

He added, "The PLO has good offices available for any constructive role in this affair."

Abdel Rahman described the PLO's move as a humanitarian gesture and asserted that the guerrilla group was acting on its own initiative. He said the move had not been discussed with the American government.

The resignation of Bazargan complicated the Carter administration's efforts to free the estimated 60 to 65 American hostages being held to demand the extradition of the exiled shah from the United States. Bazargan's provisional revolutionary government had been growing steadily weaker since he was installed by Khomeini in February after the collapse of the monarchy.

Bazargan said interference from the Moslem clergy had made it impossible for him to govern.

His resignation, prompted in part by domestic criticism for a meeting he held in Algiers last week with U.S. national security adviser Zbigniew Brzezinski, stripped away the last facade of civilian government cloaking Iran's increasingly authoritarian clerical rule. It also presented Washington with the task of having to try to negotiate with clergymen in Khomeini's entourage who neither understand nor care about the rules of international diplomacy.

In accepting the resignation of Bazargan and his ministers, Khomeini said, "I have assigned the Revolutionary Council to run the affairs of the country during the transitional period."

The secret council, consisting mostly of Moslem clergymen staunchly loyal to Khomeini, has been Iran's defacto executive power since the revolution.It formally took the reins from a government that was too weak to handle such incidents as the takeover of the U.s. Embassy by Iranians claiming to be students.

Shortly after the Bazargan government's resignation was announced, the Revolutionary Council asked the ministers to continue carrying out their duties for the time being, according to the official Tehran radio.

In another broadcast, Khomeini said from his residence in the holy city of Qom, "Some people are now pressing that students should leave that embassy. But we cannot sit idlehanded when the United States is weaving conspiracies against us. We demand that the United States should extradite the criminal shah to us, and the British government should extradite the criminal Bakhtiar."

Khomeini was referring to Shahpour Baktiar, the shah's last prime minister, who in fact is living in France.

Khomeini added, "We shall take other steps if this is not done and if these criminals are not expelled from those countries."

No extraditon treaty exists between the United States and Iran, and U.S. sources have said that any effort by Washington to deport the shah could be blocked by a U.S. court.

The 79-year -old Khomenini instructed the Revolutionary Council to prepare the for a referendum on Iran's new constitution and for elections of a president and parliament.

All three events, however, appear to be still weeks away in view of continuing debate on the draft of the constitution in a special consulting group called the Assembly of Experts.

In the interim, it seems probable that the Revolutionary Council will put together a new Cabinet, but whether it will include ministers of the outgoing government is unclear.

Members of the Revolutionary Council have never been officially named but are believed to include officials of Bazargan's Cabinet, among them Foreign Minister Ibrahim Yazdi and Defense Minister Mostafa Ali Charmran.

It is also possible that the Revolutionary Coucil will simply seek to exercise direct control over the adminstration, observers said.

There was no immediately obvious public reaction to Bazargan's resignation, which removes the last barrier between the clergy and direct control of national affairs.

The change is likely to be applauded by Khomeini's more committed followers, but will clearly increase apprehension among middle class moderate and seculr groups about the prospect of complete clerical control.

In a brief statement issued today, Khomeini acknowledged Bazargan's "back-a brief statement issued today, Khomeini acknowledged Bazargan's "back-breaking labor" and his "piety, honesty and good intention."

But his acceptance of the prime minister's resignation, which has been tendered several times before, reflected the fact that Bazargan had become a political liability to the ayatollah.

In recent months there had been ever sharper criticism of Bazargan's administration for being "insufficiently revolutionary" and for its failure to implement the policies advocated by the clergy and Revolutionary Council.

Bazargan, a liberal, had considered his main objective to be stabilizing the country and restoring calm in preparation of elections of a new government.

Examples of his failing cited by critics included a reluctance to purge the administration completely of those associated with the shah's government and the adoption of relatively conservative economic policies.

Bazargan was particularly compromised by his meeting with Brezezinski, which coincided with an upsurge in anti-American sentiment triggered by Khomeini's bitter condemnation of the United States of admitting the shah for medical treatment.

The anti-American campaign has proved a valuable instrument for rallying public support for the clergy's rule at a time of mounting disaffection caused by the regime's lack of progress in tackling social and economic issues. c

It also has served to eclipse at least temporarily the deepening rifts between members of the Cabinet and the Revolutionary Council over such issues as the constitution and the strategy for setting the Kurdish revolt.

In his letter of resignation, Bazargan told Khomeini tht "interference, meddling, opposition and differences of opinion" led to his decision. "The discharge of our duties and continuance of our responsibilities have for some time been impossible," he wrote.

Khomeini's office in Qom banned demonstations scheduled for Wednesday in support of Bazargan by a previously unknown group called Young Crusader Moslems. The militants holding the embassy charged that the demonstrations were planned by the shah's disbanded secret police SAVAK and the CIA in Bazargan's name.

The militants at the embassy emphasized again today that the return of the shah was the only thing that could bring about release of the hostages, and they warned that they would kill their U.S. captives if the United States attempted any direct military or covert act to secure their release.

The militants said that the hostages were being well treated, but reneged for the second successive day on a promise to let reporters see them. Such a meeting will take place they said, but declined to fix a time.

The militants said armed guards around the hostages had been reinforce and that restrictions on their captives had been relaxed.

"All the hostages are well and free to move around" in a restricted area inside the embassy compound, a spokesman for the occupiers said. "They are not blindfolded anymore."

A statement by the militants broadcast on the government radio ordered the U.S. charge d'affaires, Bruce Laingen, to surrender to them. Laingen was not in the embassy when it was seized Sunday.

Asked what would happen if U.S. officials arrested Iranian citizens in the United States in retaliation for their action, a spokeman for the militants said, "There are Americans outside the embasy who are not hostage. We will fight against any compromise on the part of the Revolutionary Council."

The militants said that Secretary of State Cyrus Vance had sent a message through the Iranian Foreign Ministry to Ayatollah Khomeini's clergyman son, Hojatoleslam Admad Khomeini, asking him to mediate with the occupiers.

Ahmad Khomeini appeared to have the confidence of the militants, who yesterday invited him for talks on the action they had taken.

They said, however, that even his mediation would be unacceptable to them and that in any case Ahmad Khomeini had declined the request.

Meanwhile, the campaign against the United States continued to gather momentum with the arrest today of two more U.s. citizens amid fears that pro-Khomeini militants were preparing to detain members of the U.S. business community as well.

Two staff members of a U.S.-sponsored cultural and educational institute, the Iran-America Society, were sized by an unidentified group and taken to join the other hostges at the U.S. embassy. They were identified as society director Catherine Cooke and director of studies William Royer.

Pan-American Sirways today withdrew its U.S. personnel from Iran, and other American executives were understood to have been advised by the Iran-America Chamber of Commerce to stay away from their offices and go underground.

[A Revolutionary Council statement broadcast by Radio Tehran Tuesday night, however, warned against "seizing and attacking foreign nationals," possibly referring to some 700 other Americans still working in Iran, Associated Press reported.]

The latest events also were accompanied by renewed action against foreign reporters,several of whom were prevented from entering Iran today.