The U.N. Security Council yesterday appealed for the immediate release of Americans being held hostage by Iranian militants at the U.S. Embrassy in Tehran as a barrage of world criticism politically isolated Iran's revolutionary government.

Coinciding with the U.N. statement was an offer by Egyptian President Anwar Sadat to defuse the issue by flying the ailing shah of Iran from the United States to Egypt. Pledges of help in negotiating the release of the more than 60 U.S. captives came from such diverse figures as Pope John Paul II and Muhammad Ali.

In Tehran, tens of thousands of Iranians demostrated in front of the U.S. Embassy on the sixth day of the takeover by Moslem students to back demands that Shah Mohammad Reza Pahlavi be returned to Iran for trial. The shah, whose monarchy was toppled in February, is being treated for lymphatic cancer and bile duct problems in a New York hospital.

The U.S. charge d'affaires in Iran, L. Bruce Laingen, discusses the hostage-taking for the first time with a member of the ruling Revolutinary Council, which runs the country in the name of Moslem leader Ayatollah Khomeini.

Representatives of the Palestine Liberation Organization also conferred with government officials in Iran and with U.S. envoys temporarily stuck in neighboring Turkey, but no progress in the efforts to end the embassy siege was reported.

The Revolutionary Council member, Abol Hassan Bani-Sadr, said in an interview published yesterday that "there is no question of executing the hostages." But, he added, "Their liberation depends on the United States." h

[A Tehran radio broadcast, issued in the name of the students holding the hostages, said ambassadors of four countries -- Algeria, France, Syria and Sweden -- would be allowed inside the embassy "to tell all the nations of the world, and especially the American nation" about the conditions there, the Associated Press reported.]

The militants who have been holding the embassy since Sunday have threatened to kill their captives if the United States tries to free them by force.

The U.N. Security Council was joined by General Assembly President Salim A. Salim of Tanzania in appealing for the immediate release of the Tehran hostages. In addition, U.N. Secretary General Kurt Waldheim reportedly was exploring the possibility of a personal trip to Iran to negotiate directly with Khomeini.

The Security Council's statement, issued by the body's president, Ambassador Sergio Palaciso de Vizzioof Bolivia, said he was authorized "to express the council's profound concern over the prolonged detention of American diplomatic personnel in Iran. $"While not wishing to interfere in the internal affairs of any country," he added, "I must emphasize that the principle of inviolability of diplomatic personnel and establishments be respected in all cases in accordance with internationally accepted norms.

"Therefore, I urge in the strongest terms that the diplomatic personnel being held in Iran be released without delay and provided protection. I further urge the secretary general to continue to use his good offices to assist towards this objective."

After the Security Council meeting, which was called by the United States, U.S. Ambassador Donald McHenry said the statement was "helpful" but that he did not know if Iranian authorities would "react positively."

In private talks, some of the 15 Council members, notably the Soviet Union and Kuwait, expressed reservations about the tactical wisdom of a public appeal and suggested that quiet diplomacy would be more effective. The statement was eventually agreed upon after two hours of deliberation.

Soviet Ambassador Oleg A. Troyanovsky said before the talks that "essentially our position is that diplomatic immunity of foreign embassies should be adhered to strictly in all cases and in all countries."

The Iranian charge d'affaires at the United Nations, Jamal Shemirani, later warned that "the council appeal cannot help and it could even inflame the situation. The best channel is the direct channel." He was referring to talks in Tehran between Laingen, the U.S. charge, and Bani-Sadr, a close Khomeini aide who has been appointed to run the Foreign Ministry following the resignation this week of prime minister Mehdi Bazargan's government.

In Egypt, Sadat renewed a standing offer of asylum for the shah and denounced Khomeini as a "lunatic" whose actions were a "disgrace to Islam."

At a news conference, a visibly agitated Sadat said, "I am really infuriated, Islam teaches love and brotherhood. It does not teach at all what this man is doing." He called on other Moslem leaders to condemn Iranian behavior that he said was "exploiting Islam" and predicted that "sooner or later the whole country will fall into the hands of the leftists."

Sadat earlier said he had a jet standing by to fly to New York and bring the shah, his doctors and family to Egypt, where the shah would be treated.

The Egyptian ambassador to the United States, Ashraf Ghorbal, flew to New York yesterday to convey Sadat's offer to the shah personally. The ailing monarch's spokesmen and doctors refused to comment on any results of the meeting.

Hospital spokesmen, however, said a flight to Egypt would be "extremely dangerous" for the shah's condition. They said that if such a trip were made, it would be against the advice of his doctors.

"If the shah left, it would be for reasons that overide medical ones," a hospital source said.

The New York Post reported meanwhile that former heavyweight boxing champion Muhammad Ali offered to go to Iran to help free the hostages.

"Even if they wanted to hold me instead of those people, I'd go," he was quoted as saying. "I'm a Muslim and I don't think they would hurt me. Even if they shoot me, I'm just one man, and if I could save 60 people, I'd take my chances."

In Tehran, Vatican diplomatic sources said Pope John Paul II has sent a message to Ayatollah Khomenini asking him to ensure the safety of the estimated 100 hostages, including non-Americans, being held at the embassy.

Washington Post special correspondent Nicholas Cumming-Bruce reported the following from Tehran:

The pope's intervention came in a personal message for Ayatollah Khomeini delivered to the Foreign Ministry by the Vatican's ambassador in Tehran, Annibale Bugnini, who returned from Rome Thursday.

at the same time, there were suggestions that European embassies may shortly make a collective approach to the Iranian authorities following the attempts to see the hostages by other members of the Tehran diplomatic corps, including the amabssadors of Iran's Moslem neighbors, Pakistan and Turkey.

Despite the growing momentum of international activity to obtain the release of the hostages, however, there has been no hint yet of any softening of the intransigent position adopted by Khomeini and the Moslem students acting in his name.

No information has yet been provided by the students on the condition of the hostages beyond claims that they are being well treated and photographs showing U.S. staff, including a Marine guard, seated in residential quarters.

Another photograph released yesterday was of the U.S. defense attache, Tom Schaeffer, seen sitting on a bed in a small room under the observation of what appeared to be an unarmed student guard.

Meanwhile, at a massive gathering at Tehran University for Friday prayers, Tehran's spiritual leader, Ayatollah Hossein Ali Montazeri, repeated harsh criticism of the United States for admitting the shah.

"For the sake of one man, the U.S. government is damaging the prestige of America throughout the world," he asserted. "What kind of government is that?

"President Carter has to understand that if he does not return the shah alive, the struggle will become harsher," he warned.

Tens of thousands of Iranians later marched from the university to the embassy in a demonstration of solidarity for the occupiers. They chanted, "America, America, death to you. Islam is victorious, the United States is destroyed."

The march, however, lacked some of the bitter animosity directed toward the United States in recent demonstrations, and quickly dispersed on reaching it destination. Several thousand people stayed to keep up the vigil that has been maintained at the gates of the embassy since it was taken over.

The invasion of embassy compound, according to eyewitness accounts only now emerging, was a restrained and well-planned operation.

The students entered the compound after forcing the main gate under a banner announcing their intention to hold a sit-in.

Their encounter with the embassy's Marine guards, reportedly was not a serious clash, although the Marines fired tear gas.

The students were said to have been polite but purposeful, even when it came to the binding of their captives.

Some two hours passed, however, between the time the students entered the compound and the moment they managed to take over the main embassy building and reach the communications room.

The time gap was apparently sufficient to allow for the destruction of sensitive documents and communications equipment.

Some seven hours after the takeover, the students released an estimated 40 Iranian employes after briefly questioning them. At that point the ordeal of the embassy staff started.

The official Tehran radio said representatives of the Iranian equivalent of the Red Cross, the Red Lion and Sun Society, visited the embassy Friday and found the hostages to be in good condition. However, a PLO source in Tehran said the society reported that some captives were suffering form "mental duress."

In Turkey, PLO representatives reportedly were in contact with former U.S. attorney general Ramsey Clark and the chief of staff of the Senate Intelligence Committee, William G. Miller.