More than a million Iranians marched through Tehran streets today in the biggest show of strength by exile Moslem Leader Ayatollah Ruhollah Khomeini designed to sweep away Prime Minister Shahpour Bakhtiar's government.

Throngs in cities across Iran marched in festive and generally peaceful parades called by Khomeini to demonstrate opposition to the embattled government. An estimated 500,000 persons marched in Masshad and 100,000 in Qom.

But there were growing fears of a coup by the armed forces opposed to the exiled Moslem leader's plan. Such a move could be set into motion if Khomeini, who is in France, were to announce his Islamic revolutionary council -- the beginning of a parallel government leading to his dream of changing the monarchy into an Islamic republic.

Even Ayatollah Kazem Shariatmadari, the conservative religious leader in the holy city of Qom, felt constrained to come closer than ever before to criticizing Khomeini's refusal to compromise with Bakhtiar.

He told reporters that if the Bakhtiar government fell he feared "more violence and trouble" and the "possibility of an army reaction."

Khomeini, in an interview with French television, called today's marches a "referendum" in favor of his Islamic republic. "The shah is already dethroned and we will take power through the legitimate referendum in the streets," he said.

In the Tehran marches, demonstrators called for an Islamic republic. They appeared subdued, even glum, compared to the only slightly smaller, but more lively marches early last month when Iranians voted with their feet against Shah Mohammad Reza Pahlavi.

Political analysts ascribe the change of political mood to a normal letdown after achievement of the opposition's year-long goal -- the shah's departure abroad only three days ago.

Moreover there are numerous uncertainties facing Iran, ranging from a threatened coup d'etat to the damaged social fabric and economy after a year of turmoil and weeks of paralyzing general strikes.

Although resolutions handed out at the end of the seven-mile-long march again refused to recognize the legitimacy of the Bakhtiar government, few if any banners or slogans hostile to him were in evidence.

Rather it was as if he had become a non-person despite many Iranians' inner convictions that his present Cabinet represented the best compromise solution to the country's growing problems.

One banner announcing "Be Careful Not to Allow Another 1953 Coup d'Etat" struck a deeply felt fear by suggesting that the shah could yet return to power much as he did more than a quarter century ago in a CIA-aided putsch.

"Yes, the shah is gone," an earnest young engineer said, "but we must get rid of the many remaining agents of the shah. We need drastic change."

The government and martial law authorities bent over backwards to avoid confrontation with the demonstrators today. They went even further than during the December marches when police and soldiers were kept away from the line of march.

This time the ranks of soldiers and tanks which in December had protected upper-class northern Tehran -- and the shah's place -- were nowhere in evidence.

Along the line of march Khomeini' portraits, large and small, black and white and in color, far outnumbered those of other religious leaders. Only minor scuffles were reported between Marxist activists and religious demonstrators.

Indicative of the changing mood were reports that residents of an upper class neighborhood in northern Tehran had been warned to march today, otherwise their names would be marked down and remembered. In the past participation in the anti-regime marches has been voluntary.

Asked what he thought lay in store for Iran, a demonstrating dentist said, "Honestly, no one knows."

An increasing number of Iranians find themselves in the uncomfortable position of criticizing Khomeini for the first time after respecting him as the focus of their own struggle to remove the shah.

Opposition professional men who only weeks ago were solid Khomeini backers are now openly worried about the prospect of replacing one autocratic leader with another.

Even some men expected to serve Khomeini politically give visitors the impression these days of wishing some last-minute compromise could be worked out between Bakhtiar and the religious leader. They fear the alternative is a military coup.

So far all missions to Khomeini have focused on explaining that only with the greatest persuasion was the shah able to win over his senior generals to accepting Bakhtiar, who in turn accepted the institution of a monarchy.

Abolition of a monarchy may yet come, Khomeini's visitors have said, but the military which looked askance only weeks ago at Bakhtiar must be given time to absorb such a bitter pill.

The possible compromise worked out by Darius Forouhar, the current emissary to Paris of the National Front opposition, suggested coexistence between Khomeini's revolutionary council and the regency council set up to preserve the monarchy's outward trappings last weekend.

But it is still unknown if he or other emissaries have gotten through to Khomeini. The ayatollah's most recent blast at Seyed Jallaleddin Tehrani, the Regency Council chairman dispatched to Paris to see him, was scarcely encouraging. Tehrani would be received, but only if he resigned from the council, Khomeini said.