Iran late last night sent a message to the United States outlining its conditions for transferring the estimated 50 American hostages to government control and appealed for postponement of threatened economic and political sanctions for the next 24 hours.

The contents of President Abol Hassah Bani-Sadr's message -- transmitted by Swiss Ambassador Eric Land -- were not disclosed, but observers suggested that Iran's fear of appearing to knuckle under to threats from Washington would leave little room for proposals that Washington would find acceptable.

But in a speech delivered by his son early this afternoon, Ayatollah Ruhollah Khomeini blasted the recent U.S. threats and said the hostages' fate will continue to rest with the parliament. He denounced the American-sponsored compromise as "diabolical trickery, [which] was an attempt to deceive us through flattery."

Khomeini, however, did not specifically mention the transfer of the hostages to the ruling Revolutionary Council's control.

Foreign Minister Sadegh Ghotbzadeh said he was optimistic about reaching agreement on the hostages transfer, but said all parties "should refrain from any [reprisal] action in the next 24 hours."

Ghotbzadeh had an apparently positive visit yesterday with Khomeini.

Informed sources said the crucial details of the transfer plan had yet to be definitively worked out. They would be announced at midday today local time when Bani-Sadr is scheduled to make what is billed as a "very important speech" on domestic and foreign issues.

Circumstantial evidence suggested that Iran had failed to meet an apparent midday deadline yesterday set by the United States in a purported message from President Carter demanding an Iranian commitment to transfer the hostages under pain of sanctions. Today marks their 150th day in detention at the American Embassy.

Significantly, leaders of the Islamic militants holding the hostages were reported locked in negotiations with Bani-Sadr that began at midday yesterday. Three of the militant student captors were seen leaving his offices late last night. They refused to make any substantive remarks.

Informed sources said the militants had proposed that the government take over the embassy, but leave them inside the 27-acre compound together with the hostages. The United States, the sources said, had rejected that proposal.

These latest developments followed a confusing weekend of claims and counterclaims by Iranian and U.S. authorities on a purported series of messages sent by President Carter to Iran.The Swiss Foreign Ministry confirmed that Lang had transmitted a letter from Carter to Bani-Sadr last week. The Iranian president said he had received a second message from Carter Sunday morning and Ghotbzadeh has spoken of receiving at least one "Oral" message via a trusted third party.

Denials by U.S. officials, meanwhile, have centered on the purported content of the messages as stated by Iranian officials, rather than on the existence of the communications.

Hopes for a resolution yesterday of the transfer question -- raised anew Sunday night with reports that a majority of the volatile Revolutionary Council favored taking the Americans under government control -- were dampened by midday.

Despite Ghotbzadeh's seemingly positive morning visit with Khomeini, whose approval is required for any such major decision, Bani-Sadr's office denied that the president planned any public statement until his Tuesday speech.

By mid-afternoon, informed sources reported an agreement on a formula. But the fact that the militant student leaders -- including at least one member of the students' central committee -- emerged only at 10 p.m. last night suggested that perhaps substantial changes had been made.

Informed sources added that a written outline of the long day's discussion had been sent to Khomeini late last evening for his perusal.

Reporters were kept away from Bani-Sadr's office during an evening meeting involving members of the Revolutionary Council.

Spokesman Hassan Habibi uncharacteristically drove out of the meeting without talking to journalists, who were kept well back from the office's street entrance.

Tehran Radio announced that Bani-Sadr had presided over a 5 1/2-hour meeting of the 15-man National Security Council -- composed of armed services chiefs and the defense and interior ministers to "examine the security situation in Iran."

Meanwhile, Iranian leaders were reported concerned with the threatened American sanctions and the apparent support that the nine members of the European Economic Community are providing.

Khomeini, in remarks to Islamic judges made Sunday and published yesterday, seemed to refer to indications from Washington to renewed pressure on Iran.

"No power can impose its will, neither America nor the Soviet Union," he said. "In a country where all are united, nothing can be imposed on such a country.

"They cry from abroad, 'We will do such and such.' If you remember, they said, for example, they intended to send parachutists and enter the nest of spies [U.S. Embassy] and take the spies. They said they intended, for example, to intervene militarily and obliterate everybody. These were words," Khomeini said in his speech. "Do not be afraid of words."

Khomeini has steadfastly insisted that the hostage crisis must be settled by the Iranian parliament, which is still in the process of being elected. The parliament is not expected to meet until late May or early June at the earliest.

Bani-sadr has staked much of his ideological reputation on advocating a foreign policy playing off Europe and Japan against the super-powers. Starting last Thursday, the European envoys accredited here have been backing up Washington's demands for the hostages' release with visits to the president deploring the crisis involving their two sets of friends -- Iran and the United States.