Iranian troops fired into the air to disperse Tehran residents who broke the martial law curfew tonight. Ambulance sirens wailed after the heavy firing but there were no reports of casualties.

Reporters, curtailed by the curfew, were unable to go to the area of the firing.

Witnesses said residents left their homes to see if the new moon had appeared, initiating the holy Moslem period of Moharram. They said the troops fired the warning shots in the southern sector of the city near the railway station, the scene of recent demonstrations against the rule of Shah Mohammad Reza Pahlavi.There were unconfirmed reports of artillery fire.

Fears mounted here on the eve of the 29-day mourning period, when religous fervor runs high and the more militant faithful whip themselves in memory of the prophet Mohammed's grandson, Imam Hossein, who was martyred in 641 A.D.

Western diplomats and conservative elements opposed to the shah said they feared the period "could turn out to be a bloody shooting match" between troops and anti-shah demonstrators.

Several reliable sources said government troops recently uncovered a cache of Soviet made assault rifles in Tabriz, a city in northwestern Iran that has been the scene of rioting for 11 months.

This followed government reports that handguns, grenades and ammunition were found in the city of Mashhad and other weapons found in the capital.

The government of Gen. Gholam Reza Azhari has banned all public religious processions during Moharram and warned that any attempts to demonstrate would be put down mercilessly.

Despite the ban, thousands of demonstrators reportedly took to the streets. Cries of "death to the shah," and "Allah akhbar" (God is great) echoed through the capital.

The demonstrations came only hours after Ayatollah Ruhollah Khomeini, 78, exiled leader of the opposition, sent a letter to his Shiite Moslem followers from Paris, proclaiming that "torrents of blood will be spilt on Ashura" -- the tenth day of mourning on Dec. 11. Shiite Moslems plan then to observe the traditional beating ceremonies despite a government ban. Khomeini called for a general strike starting Saturday.

"Blood will triumph over the sword," his letter said.

"People of Iran, sacrifice your blood to protect Islam and overthrow the tyrant and his parasites! Pay no taxes! The tree of oppression will be cut down! These are critical days for Islam: you will be remembered for your sacrifices," Khomeini said.

His tone caused dismay among Americans and other members of Iran's foreign community, many of whom have already fled the country and its growing civil strife. In Tehran, panicky residents began hoarding food and gasoline and braced for bloodshed.

Khomeini ordered his followers to prepare a list of all the shah's "henchmen" so that they could be punished for complicity with the government.

His call for war waged in the name of Islam appeared to dim hopes for the settlement that Jordan's King Hussein was reportedly trying to mediate between the shah and Khomeini. Arab newspapers reported that Hussein planned to travel to Paris to see Khomeini.

The mourning ceremonies took on political overtones when the Shiites bestowed holy martyrdom on more than 1,000 demonstrators killed in the past year of violence.

Although the government banned the Moharram ceremonies this year, the powerful Shiite clergy that Khomeini leads planned to hold them anyway, setting the stage for a showdown with the military.

"We don't need permission for practicing religion," one clergyman said.

Conservative religious leaders and opposition politicians have spearheaded the movement against the 49-year-old shah because they oppose his West-tern-style reforms or find fault with his authoritarian one-man rule.

Although Khomeini has vowed to topple the monarch, a dissident source conceded yesterday that if the army can maintain a semblance of order during the first three weeks of Moharram, "the shah will probably be home free and through the crisis."

The civil unrest has presented the shah with the most serious threat since he took over from his father in 1941.

In his strike call, Khomeini appealed to the oil industry workers to walk out again. The 37,000 Iranian workers walked out in a crippling strike from Oct. 31 to Nov. 14, cutting off the nation's prime source of income.