The Security Council today set Saturday night for the start of a full-scale debate about the U.S.-Iranian confrontation, testing whether the United Nations can play an effective role in winning release of the 49 Americans held hostage in Tehran.

Following 2 1/2 days of intense behind-the-scenes maneuverings, the 15-nation council agreed to begin the debate at 9 p.m. Saturday after receiving assurances from Iran that its acting foreign minister, Abol Hassan Bani-Sadr, will be present to state his country's position.

Bani-Sadr's expected arrival here raised the possibility that Secretary of State Cyrus Vance would lead the U.s. delegation. Since the beginning of the crisis, U.S. efforts to obtain the hostages' release have been hampered by the absence of direct contact with Ayatollah Ruhollah Khomeini or those close to him.

U.S. sources here cautioned that it was not clear whether Bani-Sadr, who is a member of the Islamic Revolutionary Council, will have any real negotiating flexibility, especially after Khomeini's statements today questioning the United Nations' competence to deal with Iranian events.

But the least the Carter administration officials hope to win is unanimous Security Council condemnation of Iran's seizure of the embassy and a call for release of the hostages. The forthcoming debate should also demonstrate U.S. willingness to explore all possible avenues for settling the dispute without resort to force.

U.s. sources say it was these considerations that led the administration on Sunday to drop its earlier opposition to Iranian demands for a Security Council debate. Previously, the administration feared that such a debate would allow Iran a forum for stating its grievances against the shah and the United States and deflect attention from the plight of the hostages.

Washington's switch came after U.N. Secretary General Kurt Waldheim apparently became convinced that U.N. efforts to work out a possible compromise were fading.

Waldheim took the unusuall step of calling for a Council meeting because he considered the situation "a threat to international peace." The United States immediately gave strong backing to Waldheim's appeal. However, both apparently acted on the basis of an announcement by Bani-Sadr that he was coming to New York immediately -- an announcement that later was rescinded by the Iranians.

As a result, there ensued protracted negotiations in which the United States insisted first on an immediate Council meeting and then, retreating somewhat, on at least the opening of a meeting with the United States being given the opportunity to state its position that the hostges must be freed immediately.

However, the U.S. push ran into opposition from nonaligned countries on the Council. They were understood to have been influenced by Iranian arguments that it would be fruitless and even dangerous for Bani-Sadr to come here until the weekend when the Moslem holiday of Muharram has run its main course.

A compromise finally patched together today saw Washington agree to delay until Saturday in exchange for formal assurances from Iran, delivered to Waldeim, that Bani-Sadr will take part in the debate Saturday night.

Under this compromise formula the Council met this afternoon to agree to Waldeheim's request for an Iran debate. In adddition, the current Council president, Sergio Palacios de Vizzio, reiterated the statement of Nov. 9 on behalf of all Security Council members urgently asking for the release and protection of the hostages.

The only other person to speak at today's session was Waldheim, who repeated his concern that the situation "threatens the peace and stability of the region and could well have very grave consequences for the entire world."

As part of the compromise, the United States dropped its insistence on making a statement today. However, U.S. Ambassador Donald McHenry, in a press conference afterward, said the United States was satisfied with the Council's action.

McHenry noted that the Council, through the statement by Palacios and Waldheim, had restated its "unanimous opinion" that Iran is acting illegally under international law by holding the hostages. He concluded by citing the "restraint shown by the American public" since the beginning of the crisis and warned: "I would suggest this restraint should not be misunderstood or interpreted as one which in any way acquiesces with the action taken in Iran."

U.S. sources said later that, given the conflicting statements coming out of Iran, they could not say with certainty whether Bani-Sadr actually will appear on Saturday. But, they added, if he doesn't come Iran will miss the opportunity to state its case and no longer will have any valid reason for delaying Security Council consideration of the crisis.

The official, who asked not to be named, spoke after Iranian Ambassador Mohammed Mokri held a news conference to describe his session yesterday with Soviet Foreign Minister Andrei Gromyko

Mokri quoted Gromyko as offering Iran "moral support" in its conflict with the United States. The Iranian said the subject of the hostages did not come up.

The unnamed Soviet official said "we owe the U.S. no favors" in the Middle East because American policy was aimed at cutting off Soviet influence there.

Any Soviet move to assist the United States would bring trouble for Moscow with Arab countries, the official said.

[In Paris, President Valery Giscard d'Eastaing said that if France had not accepted Ayatollah Ruhollah Khomeini as a political exile last year, the French probably would be in the same position as the Americans today. He described French relations with oil-exporting countries as having "a different tone," facilitating contracts for larger supplies of fuel.]