Key supporters of Ayatollah Ruhollah Khomeini today expressed shock, dismay and downright fear at the Moslem leader's decision to end his 14-year exile and return here Friday despite their advice to the contrary.

With crucial negotiations with senior military officers far from complete, opposition sources close to the talks expressed fears that his return now could drive the army into the very thing the opposition seeks to avert -- a coup d'etat.

The talks are in such an early stage, moreover, that Khomeini's decision "risks complicating our task," the sources said.

Their aim, they stressed, is not so much to arrange for his return, especially not so soon as to try to win over the armed forces to the idea of tolerating his demand for abolishing the monarchy and instituting an Islamic republic that the commanders consider treason.

Earlier this month Shah Mohammad Reza Pahlavi had to talk long and hard before persuading his military commanders to support Prime Minister Shahpour Bakhtiar after the shah's departure, arguing this was the best way to keep the chances of the monarchy alive.

Although diplomatic sources reported "some progress" in the talks, they stressed that "considerably more time" is needed and added there is "no chance" the negotiations can be concluded successfully by Friday.

Opposition sources suggested that key intermediaries in the indirect talks are retired colonels and generals forced out by the shah earlier for their dissenting views.

Also believed involved in the talks is Mehdi Bazargan, a 73-year-old opposition leader recently entrusted with ending crippling strikes in the oil fields.

Bazargan is expected to be on the seven-man Islamic revolutionary council and is a leading possibility to head the provisional Islamic government. Both bodies have been announced by Khomeini -- although their makeup remains secret -- and are to be entrusted with leading Iran from monarchy.

Attempting to explain why Khomeini disregarded their advice to stay in France for the time being, a source close to the talks said: "As usual, he is trusting to God.

"For the past five, six months Khomeini has been making decisions that logically no one could understand or agree with," he said. "But in an astonishing way, he has proved right."

The source explained that the plan had been to have Khomeini return only after the resignation of Bakhtiar and the Regency Council, set up to preserve the outward trappings of the monarchy after the shah left Iran last Tuesday.

"Then Khomeini could come when we have power and know the army is under control," the source added. "His decision [to come Friday] seems risky. less judicious, but he has been very bold and risky."

With one member of the nine-man Regency Council out of the country, two others -- former chief prosecutor Abdol Hossein Aliaadi and former fiance minister Mohammed Ali Varesteh -- were reported by friends to have decided to resign without yet making their decision known officially.

Bakitiar denied the reports in an interview with the British Broadcasting Corp. He said, "I'm not going to be moved by these pressures. Neither will I abandon my post. I am the legitimate ruler of Iran and continue to defend my post."

But despite this spirited self-defense, political analysts believe the real focus of power has shifted to Khomeini and his followers' negotiations with the military.

Increasingly, analysts are convinced that Bakhtiar may already have served his purpose -- to get the shah out of Iran by reassuring him and the generals that he would not demand abolition of the monarchy, which they found unacceptable.

Great importance is attached to the presence here of Lt. Gen. Robert Huyser, the U.S. Ar Force officer who arrived more than two weeks ago from West Germany, where he served as the American deputy to the North Atlantic Treaty Organization commander. His reported mission is to keep telling his Iranian counterparts that moderation -- not a coup d'etat -- is needed to forestall a communist takeover of Iran.

The generals "must have the backing of one superpower or at least its assent for any coup to succeed," said one veteran diplomat.

He wondered out loud, however, whether they understand that reality.

Significantly, Soviet Ambassador Vladimir Vinogradov recently has been reminding diplomatic colleagues of a 1921 treaty that gives Moscow the right to intervene if Iran were to become a military base against the Soviet Union.

Meanwhile, a demonstration of between 5,000 and 10,000 leftsts -- ranging from socialists and Marxists to a small group of orthodox Communist Party members -- marched through Tehran's streets today, harassed and taunted by Khomeini followers who were visibly unimpressed by their shouts of "Honor to Khomeini" and "unity," as well as distinctly leftist slogans.

The Khomeini Moslems shouted: "You leftists. You are traitors. You are trying to sabotage the revolution. We should preserve our unity."

In an open letter to Khomeini published in newspapers, a Marxist guerrilla group warned against the Moslem "attempt to monopolize the revolution" and groups seeking to "use Islam as a pretext to revive the Inquisition" and enforce a single party.

The U.S. Embassy said meanwhile, that an assistant defense attache, Navy Capt. Ray N. Hollingworth, was shot at, but escaped injury, last night in his Tehran home.

In the last month, two Americans have been killed in a growing spate of anti-Americanism, and the U.S. community, once more than 4,000 has shrunk to about 12,000.