Shells and rockets pounded commercial and residential neighborhoods of Christian East Beirut early today as Christian militiamen and Druze fighters in the hills overlooking the capital exchanged hundreds of rounds of artillery in the predawn darkness.

At least two persons were killed in the shelling--one in the hills and one in the city--and there was scattered damage to buildings and cars in the worst bout of violence in Beirut since the Israeli siege last summer.

In a separate incident later this morning, an Israeli soldier was killed and four others were wounded when hit-and-run gunmen fired rocket-propelled grenades at their tank and armored-personnel-carrier unit as it patrolled just south of the city near the Galerie Semaan crossing point between East and West Beirut.

State and privately owned Lebanese radio and television reported that an elderly woman, the mother of a Lebanese Army colonel, and another unidentified person were killed when the Israeli patrol retaliated by leveling a nearby two-story house with machine-gun fire and tank shells.

The grim Sunday increased fears in this city, where acts of violence are often followed by acts of vengeance, that the fragile calm that has existed in recent months here was deterioriating.

"Why? Why? It's useless to ask why," said an East Beirut woman after enduring a morning in which Druze militiamen fired more than 150 rounds into the heart of East Beirut and its suburbs--in retaliation, they said later, for the 500 rounds that the Christians fired into Druze villages in the mountains.

"What can you do?" asked a laborer from India as he pointed to a three-foot shell hole in a parking lot just 25 feet from the crude cinderblock shed where he and fellow laborers had been sleeping.

The shell destroyed two Mercedes automobiles parked on the lot and blasted out the windows of the shed, the adjacent bank building that the laborers had been remodeling and apartments across the street.

In the artillery exchanges, Druze militiamen said that one unidentified civilian was killed and two others were wounded in villages in the hills. They also said there was damage to mosques and many homes.

In East Beirut, the shelling also killed a 70-year-old French priest who stepped out on his balcony when he heard firing and was killed by the third of three shells that hit the chapel of his monastery.

The French Embassy said the priest, Clothaire Semeux, had lived in Syria and Lebanon since 1939 and was with the Brothers of Lazarus order in the Ashrafiyeh section in the heart of East Beirut.

West Beirut was the scene of three bombings on Thursday and Friday. On Saturday a motorcycle rider in the predominantly Moslem section of the capital hurled a grenade at a French troop truck, wounding one soldier of the international peace-keeping force.

Today, however, West Beirut was calm, with huge traffic jams in the downtown business district, joggers on the streets and crowds strolling along the seafront.

The Lebanese Forces Christian militia and Druze fighters blamed each other for beginning the four-hour artillery duel between positions in the hills and East Beirut.

Christian militiamen, who had a press release prepared for reporters who came to their sprawling military compound next to the port of Beirut, said the Druze had been assisted in the artillery battles by Syrian guns farther east in the mountains. The press release identified places hit in East Beirut and the locations where they said the firing had originated.

Christian militia press aides provided escorts to reporters to inspect the damage. One escort explained that much of the debris from the shelling had already been cleaned up by on-call crews that had been summoned to the streets earlier in the morning.

At a Druze command post in the back room of a former curio shop in Chouyeifat, a village about five miles from Beirut, militiamen heatedly denied that they had received help from the Syrians.

"This is all kind of BS," said a man who did translation for a Druze officer identified as Abu Jamil. "Here we have no Palestinians, no Syrians, only Druze. Druze is a sect known for their toughness and they fight only for their dignity. We have no intruders around here."

" The Christians shell at us, so we must reply to the place where the shelling had come from," the man identified as Abu Jamil said.

There has been fighting off and on between Druze and Christians in the Israeli-occupied hills overlooking Beirut since last fall. The Druze, a breakaway Islamic sect, have been angered that Christian militiamen, de facto allies of Israel, established barracks and checkpoints in the hills after the Israeli invasion.

The Christian militia said they went there to provide protection for Christians returning to their hill villages where most of the people are Druze.

Israeli officers have been accused of arming both sides in the conflict, a charge that Israeli spokesmen deny. Israeli officers had been mediating the dispute in recent weeks, and hostilities had lessened.

Until today, the Druze had refrained from shelling East Beirut, but they claimed Christians were firing on their villages with artillery based in the capital.