Rebellious Air Force cadets, supported by thousands of armed antigovernment civilians, fought pitched battles against armed forces loyal to the shah today as Iran teetered on the brink of a bloody civil war.

More than 100 persons were reported killed in the day-long fighting, which began when the "Immortals" Division of the departed shah's Imperial Guard opened fire on cadets demonstration at an air base in East Tehran in support of Moslem leader Ayatollah Ruhollah Khomeini.

Hundreds more were wouded as the two sides exchanged machile gun and automatic rifle fire in residential areas surrounding the base. The shaky government of Premier Shahpour Bakhtiar pushed the curfew back to 4:30 p.m. but fighting continued late into the night in widely separated areas of the city, making it difficult to determine casualties.

By nightfall, the Imperial Guard had failed to dislodge the cadets and civilians from the Doshan Tapeh base, although heavy weapons fire -- including recoilless rifles and artillery -- raised the question of how long they could hold out.

Iranian military helicopters evacuated 50 to 75 American advisers working at the base, the State Department said, adding that it had advised all Americans in Tehran to remain in their houses.

Among the dead was veteran Middle East correspondent Joe Alex Morris Jr., 51, of the Los Angeles Times, who was shot crouching in front of a window in a building where he and three other American reporters sought refuge from a crossfire between the opposing forces.

The fighting was the first open split between branches of the military in the more than year-long civil unrest. It was still unclear whether the forces loyal to Shah Mohammad Reza Pahlavi and Bakhtiar would be able to restore order or whether today's fighting was the beginning of a civil war that both sides have said they feared.

It was also uncertain whether Bakhtiar would be able to hold his tenuous grip on power or if the armed forces generals would attempt a coup d'etat, which inevitably would lead to further armed resistance.

As of late this afternoon, the fighting appeared to be limited to elements of the 20,000-man Imperial Guard, led by the 5,000 "Immortals," and the Air Force cadets with a sprinkling of officers loyal to Khomeini.

Although there were cases of individual defections among the military and police, so far there was no evidence that the Army, Navy or the bulk of the Air Force were taking part in the fighting. None of the Air Force's 341 combat aircraft was involved today.

There are 220,000 troops in the Army. The Air Force has 100,00 men and the Navy 22,000.

One of the most disturbing developments for the prospects of restoring order was that for the first time Khomeini's supporters used weapons on a large scale, including Iranian-made G3 automatic rifles, AK47 automatics guns. Some of the weapons were passed out at the Air Force training base by the rebellious cadets.

Police said eight of the capital's 20 police stations had been captured or destroyed by armed Khomeini supporters.

At least 15 police officers or soldiers at the stations were killed, including six who were set afire, news agencies reported. A police source said in most cases police blew up their ammunition supplies before the stations were seized.

ospitals were in chaos, overflowing with casualties. Docotors said malny of the more than 100 dead were soldiers. Streets were littered with charred bodies and pools of blood.

However, the opposition forces were heavily out-armed by the army, which used tanks, armored personnel carriers, jeeps and turcks mounted with heavy machine guns.

Helicopter gunships criss-crossed the city throughout the day, drawing fire from the rebels and returning it. Late last night the crump of what appered to army bazookas and mortars could be heard.

Defiant civilians set fires in virtually every section of the city and by the end of the dat a black pall of smoke hung over parts of the city.

The city of 3.5 million echoed eerily with wails of protest as countless people screamed at the soldiers from rooftops, nearly drowning out the deafening roar of the shooting.

Groups of defiant rebels roamed the streets shouting the haunting rallying cry of the revolution -- "Allah Akbar" (God is great).

Khomeini, in a statement, stopped just short of calling for a jihad (holy war), but he urged his followers to "remain equipped and prepared," and to ignore the curfew.

Khomeini said, "I cannot stand these atrocities' and warned that "if the killing of the brethren does not stop and the (Imperial) Guards do not return to their own place... I will make my final decision, God willing."

Diplomatic sources said military commanders met during the day with Prime Minister Shahpour Bakhtiar and the provisional prime minister named by Khomeini, Mehdi Bazargan. The talks broke down before a ceasetire could be agreed upon.

Bazargan, appointed by Khomeini last week to head the planned Islamic republic, was said to be so shaken that he talked about reigning.

Normally busy Damavand and Farahbad streets which sandwich the sprawling air bas were laced with a crazy-quilt pattern of bunkers fashioned from sandbags in preparation for an all-out assault by the Imperial Guard.

As troops moved through side streets -- spraying the cadets and civilian guerrillas with machine gun fire, thousands of terror-stricken residents dove for cover and fled into nearby homes and offices.

Among them was Morris, the Athens-based Los Angeles Times correspondent who, while covering conflicts in the Middle East since the 1967 war, developed a reputation among his colleagues for considerable risk-taking in pursuit of a story.

With Morris at the time of his death were Washington Post Correspondent William Branigin, Chicago tribune correspondent Raymond Mosely and Arthur Higbee, Paris bureau chief of United Press International.

Branigin said Morris was hit in the chest wih a single bullet after soldiers advanced down the street with a truck mounted with a heavy machine gun toward the main gate of the air base. Branigin said it was impossible to tell where the bullet came from, but he said that just before the shotcadets and civilians had been throwing rocks at the soldiers from behind parked cars.

Higbee said one bullet had struck near the window of the second-floor photo lab where the four journalists sought cover, and all four dove to the floor. Higbee said Morris then started to rise in a crouch to look out the window, and was "caught full in the chest." Taken to a nearby hospital, he was pronounced dead on arrival.

The Imperial Guard attack on the air base began shortly before midnight Friday when, according to several cadets interviewed, some of the trainees watched a television account of Khomeini and began demonstrating in support of the ayatollah.

Of all the military branches in Iran, the Air Force has shown the Most support for Khomeini. Hundreds of relatives of Air Force personnel arrested in previous protests have staged sitins at the Justice Ministry, and Air Force recruits have been seen marching in recent pro-Khomcini parades.

Withesses said the military police ordered them to return to their barracks and go to sleep, but when some of them started to leave the compound, shooting erupted.

One cadet who joined the rebels in the street fighting said in an interview that a clonel of the Immortals Division ordered an Air Force guard to shoot the airmen, and that when he fefused to obey, the guard was shot dead by eimperial Guard troops.

Witnesses said the cadets fell back, grabbed weapons and began a fierce firefight that drove the Imperial Guard troops out of the compound.

Throughout the afternoon, cadets and civilians erected barricades around the base, sealing off block after block.

Groups of rebels, some armed only with machetes and butcher knives, guarded entrances to side streets, while sentries with rifles manned rooftops.

In the street below, eight-and nine-year-old children frantically dug up gardens and filled sandbags to build fortifications.

Automobiles and trucks loaded with rebels raced wildly through the streets, their drivers frequently holding a rifle out the window. Some soldiers were seen openly defecting to the opposition, taking their weapons with them.

One brown-robed mullah was seen walking toward the barricades carrying an automatic rifle, although some other mullahs sought to restore order and urged people not to take up arms.