Troops today shot and killed at least 15 demonstrators defying a renewed ban on public assembly here in a signal to Ayatollah Ruhollah Khomeini that the government and armed forces mean business.

As more than 100,000 Iranians demonstrated against his government here, and other crowds followed suit in the provinces, Prime Minister Shahpour Bakhtiar said in a telephone interview, "One cannot go through the apprenticeship of democracy without paying a high price."

The death toll in Tehran alone was reported to be as high as 40 by hospital sources and much higher by some demonstrators near Tehran University, where the shooting took place.

The wounded were estimated in the scores.

At least a half dozen demonstrators were reported killed in disturbances in Sanandaj in western Iran and in the northwestern city of Tabriz where 600 persons were reported arrested, according to the state radio.

Bakhtiar is determined to prevent Khomeini's return from exile unless he abandons plans to form a rival government and tolerates at least some recognition of democratic processes to achieve his aim of an Islamic republic. If nothing else, Bakhtiar has stopped Khomeini's once seemingly irresistible momentum.

With all Iranian airports still closed by government order at least through Saturday night -- and quite possibly beyond -- Bakhtiar appeared momentarily to have the upper hnd.

Analysts, however, assumed that sooner or later, Khomeini is going to return from his 14 years of exile and press for turning Iran from a monarchy into an Islamic republic.

In Paris, Khomeini repeated earlier statements that he would return to Iran Sunday unless the airport shutdown were extended. Otherwise, aides said, the ayatollah has "alternative plans" for getting back to the country.

In an interview with French radio, Bakhtiar said Khomeini's plan to return to Iran Sunday was "premature."

"The treacherous, illegal government has prevented my return. I will go back immediately after the airport is opened. I will show Bakhtiar that his government is a transgressor and will not be tolerated."

The government tonight defused a potentially more explosive situation by authorizing a mammoth pro-Khomeini demonstration planned Saturday, ostensibly to mark the anniversary of the death of the prophet Mohammed.

The hard line toward Khomeini, according to insiders, was hammered out earlier in two national security council meetings. Faced with Khomeini's announcement last Saturday that he was returning before his supporters could negotiate a compromise with Bakhtiar and the armed forces, the council pledged to arrest anyone involved in the provisional government Khomeini has announced, but so far has not made public.

Still unclear was the fate of negotations between Khomeini supporters here and the armed forces and between Bakhtiar and the ayatollah in Paris.

Fresh indications of the new tough line, which some analysts speculate may have been forced on Bakhtiar by hawkish senior military commanders, emerged last night.

Within hours of the government's renewed ban on public meetings of more than three persons, theoretically in force since martical law was instituted in September, seven journalists were arrested despite Bakhtiar's pledge to uphold freedom of the press.

Reasons for the arrests were not clear. But some of those arrested were either key figures in the newspaper union or editorialists who had criticized the army, or both.

Army behavior throughout the day varied from shoot-to-kill orders around the university to forebearance at the entrance to the access road leading to the military-occupied airport, which has been closed since Wednesday.

Tens of thousands of Iranians, many from the provinces, occupied the Shahyad monument and the highway leading to the airport access road.

There, an officer with a bullhorn tirelessly explained to the crowds that Khomeini was not arriving today. Rumors that he was to arrive had swept the country, aparently fueled by a report broadcast by Radio Moscow.

Among the tens of thousands gathered at Beheszt Zehra cemetery 20 miles south of Tehran, a shrine for those killed over the past year, many were certain that Khomeini had arrived already at Shahrokhi Air Base.

Located about 200 miles south of Tehran, the base, according to unconfirmed reports denied by the armed forces, is a hotbed of opposition activity and defiance of military discipline.

The day's violence centered around Tehran University where demonstrators, mainly students toughened by months of taunting troops, refused army requests for them to disperse.

Soldiers from the Persian Gulf port of Khorramshahr first fired in the air, and then shot to kill. Shooting continued into midafternoon as demonstrators taunted troops, who chased them into side alleys with assault guns blazing or kneeled to take careful aim at them.

Simon Dring, television correspondent for the British Broadcasting Corp., reported that he was standing next to a man shot through the head as he advanced up Shahreza Avenue in front of the university. He reported near hysterical demonstrators dipping their hands, newspapers or Khomenini pictures in the blood of the dead and the wounded, then running around stirring up other demonstrators.

Demonstrators and correspondents seeking refuge in a boarding house said the women living there kept sobbing and crying, "We are destroying ourselves," as the shooting went on outside.

Caught in an army charge, I ran down a side street where I was welcomed by a kindly man who offered a cup of tea as if nothing had happened.

A new, ugly mood was discernible among the crowd, amid scurrying ambulances, burning tires and paramedics appealing for A and O type blood.

For the first time the demonstrators shouted, "Death to Bakhtiar." Anti-American slogans, which had disappeared largely since Shah Mohammad Reza Pahlavi left the country 10 days ago, suddenly reappeared: "Death to Carter" and "Death to Bakhtair, the American dog," the protesters said.

"If our leaders let us, we will kill all Americans," said a soft-spoken freshman English major named Mohammed. "We must eradicate all after effects of imperialism and absolute monarchy. The shah was a Central Intelligence Agency pawn."

Another student said, "We are fighting American soldiers, not Iranians." When it was pointed out that the troops looked Iranian, he said, "You don't understand. What I mean is that their minds are controlled by the generals who themselves are controlled by the Americans."

Other noticeably more militant slogans included: "We are going to take arms like the Palestinians," "Only way to freedom is through armed struggle" and "Yesterday the army was our brothers, now they are killers. We need machine guns to fight them back."

By the end of the afternoon, only an occasional shot could be heard.