Three army tanks today blocked the entrance to the Tehran airport, and officials said that exiled Moslem leader Ayatollah Ruhollah Khomeini would be barred from returning as he had planned on Friday.

A police major told reporters at the roadblock, which consisted of three British-made Chieftain tanks, that "Khomeini is not coming, not at all."

Senior army officers denied any coup was involved, but troops in trucks were seen arriving in large numbers at the airport. Elsewhere the city was normal with few troops in sight.

It was not immediately clear what motivated the decision to close the airport and bar Khomeini's return. There was no official explanation for why the decision to close the airport had been taken.

Only recently Prime Minister Shah-pour Bakhtiar announced that Khomeini was welcome to return. However, Bakhtiar has made it increasingly clear in recent days that he refuses to cave in to Khomeini's demands that he resign to pave the way for an Islamic republic.

In Paris, a spokesman for Khomeini said the religious leader still plans to fly to Iran despite the airport closing. He said Khomeini will turn back only if every airport in the country is closed, news services reported.

Observers predicted an enormous explosion of public anger and quite possibly renewed and widespread violence unless Khomeini were allowed to return to Iran on Friday as planned.

Yesterday, meanwhile, the key opposition leader in Iran appealed for the government's resignation asserting that the armed forces are unwilling to use force to prevent the regime from collapsing under strikes and demonstrations.

At the first news conference since he has become Khomeini's principal envoy here, Mehdi Bazargan appealed to Bakhtiar to "take the most reasonable way and resign" before being abandoned by his ministers.

Bazargan, expected to play a major role in any transitional government, recalled his 34-year friendship with Bakhtiar and praised him as a "reasonable, logical and patriotic man" who should now give way to the ayatollah, who was scheduled to return Friday from Paris after a 14-year exile.

Bazargan argued that the "army will not risk a coup to help Bakhtiar." He said that after Shah Mohammad Reza Pahlavi's departure last Tuesday, "the generals promised not to stage a coup despite the army command's reiterated formal pledge to support the prime minister's constitutional government."

At face value, Bazargan seemed to be hinting at progress in indirect talks between himself and other Khomeini supporters and armed forces commanders designed to avoid any showdown that could destroy the last functioning vestiges of state authority.

Without further elaboration, he said "friendly and unofficial" contacts also are under way between Khomeini's agents here and members of Bakhtiar's government.

Bazargan for the first time confessed that Khomeini's declared intention to return home Friday "worried" his friends, who had hoped he would come only after a transitional government replaced Bakhtiar. But he was prudently optimistic about the home-coming itself, which millions of Iranians are expected to attend.

"We have reason to believe there will be no problems," Bazargan said, "unless the regime creates intrigues and provocations."

Faced with the prime minister's reiterated refusal to resign -- and reports he is ready to arrest anyone setting up anything smacking of a rival government -- Bazargan argued that the revolution "derives its power from the people" and Bakhtiar's Cabinet is illegal because it was appointed by the shah.

In any case Bazargan declared, some ministers have resigned, others are considering resigning, the chairman of the Regency Council has quit and two other members are about to, and government employes refuse to work for Bakhtiar.

"The only thing Bakhtiar can do is ask for the army's support," Bazargan said, in expressing hope the military "will support the people to avoid clashes."

He admitted, however, that "this is not 100 percent certain."

Turning to the yet unappointed Islamic revolutionary council and provisional government that Khomeini wants to replace the Bakhtiar Cabinet, Bazargan revealed two significant concessions, designed to mollify Iranians frightened by the theocratic implications of Khomeini's dream of changing Iran from a monarchy to an Islamic republic.

Bazargan said a referendum would be held to allow people to choose what form of government they want. At times in the past, Khomeini has brushed aside suggestions of such a referendum. His lay political opposition partners in the National Front have made such a referendum a key element in their program.

Bazargan also mentioned a "new modification," changing the nomenclature of the proposed new government from "Islamic republic" to "Islamic democratic republic." This is likely to be welcomed by the nonreligious opposition to the shah, ranging from lay liberals to Marxists.

He said he was "likely" to be part of the revolutionary council, whose membership will be announced "either Friday or shortly afterward." Neither Bazargan nor Khomeini has spelled out what they want an Islamic republic to be.

Rejecting such models as Libya or Saudi Arabia, Bazargan said he favored a government like that "we had for 10 years under the Prophet Mohammed and for five years under the Imam Ali" almost 1,300 years ago.

Khomeini was quoted in an interview with the newspaper Ettelaiat as denouncing dancing and the movies and limiting his promises of freedom of speech to exclude "things not in the national interest."

As for Marxists and other leftists, they "will be free to pursue their aims," he said, but with proviso that they did not violate the "national interest."

Newspapers had reported that Bakhtiar might shut down Tehran airport for four days starting Thursday unless Iran Air resumes full service. This dispute stems from the prime minister's contention that the strike-bound Iran air, a government carrier, cannot send an aircraft to Paris to pick up Khomeini and continue to remain closed to regular travelers.

In Paris, Khomeini aide Ibrahim Yazdi said plans to fly the ayatollah home in an Iran Air plane had run into difficulties "from officials in Iran" and talks are under way with Air France, the Associated Press reported. Yazdi added that he did not expect the trouble to delay the home-coming.

Against a background of continuing deterioration and clashes between pro and anti-shah demonstrators in Iran, the British Royal Air Force announced plans to evacuate 90 Britons, 100 Americans and more than 40 other foreigners on Wednesday from the southern oil center of Ahwaz to Bahrain, across the Persian Gulf. The Americans were mostly contract oil workers. The Britons were involved in training Iranians to care for the British Chieftain tanks in service in the Iranian army.