The U.N.-sponsored cease-fire agreement between Christian militia forces and Palestine Liberation Organization guerrillas in southern Lebanon collapsed today under the pressure of continuing sustained artillery battles between the two sides.

Stunned residents of this small hillside village prepared to bury four persons killed in a heavy barrage of shelling yesterday from the PLO stronghold in the Crusader-era Beaufort Castle, three miles across the Litani River.

Christian militia leader Maj. Saad Haddad warned that the guerrillas had obtained Soviet-made T34 tanks and were preparing to move them south from the coastal port city of Sidon.

[Israeli military sources in Tel Aviv, quoted by The Associated Press, said the Soviets gave about 60 tanks directly to the PLO, as well as armored personnel carriers and mobile artillery pieces with a range of four to six miles.]

The dull thump of mortar fire and the distinctive reports of 122mm artillery echoed through the valley beneath Beaufort Castle though the day as anxious residents gathered around damaged houses and demanded intervention by Israel.

"Why don't you take that place?" Sina Ahub, an elderly Lebanese woman shouted at an Israeli Army officer escorting several reporters into the area that took the heaviest damage yesterday. "Why don't the Israelis go straight to Beirut?" she asked.

Israel has warned that it will respond to any Syrian moves disrupting the uneasy status quo in Lebanon that endanger the Christians. The Israeli warning came after Syria unexpectedly indicated its intention to withdraw its Army troops that have helped maintain the peace here since they ended the civil war in 1976.

There was little outward evidence of Haddad's Israeli-supported militia in this part of the five-mile-wide Christian enclave that stretches 60 miles from the Mediterranean to the foothills of Mt. Hermon. But militia mortar squads were busy in the cover of the nearby olive groves, sending mortar shells in the direction of the almost impregnable Beaufort.

For its part, the PLO pointed its artillery in the vicinity of Marjayoun, where Haddad makes his headquarters, although damage was reported slight. Israeli Army officers said the road from Qlaias to Marjayoun came under sporadic fire from the guerrillas.

Dir Mimas, a predominantly Moslem village of 1,500 about three miles north of the Israeli border, took the brunt of yesterday's attacks, in which four persons, including a 12-year-old girl, were killed and four other residents wounded.

In the predawn darkness, guerrillas slipped down the ridge from Beaufort, crossed the shallow Litani by foot and placed explosives against five houses. Israeli Army officers pointed out the route the terrorists took noting bitterly that it passes alongside a Nepalese contingent of the United Nations peacekeeping force encamped in the valley.

In one of the houses, owned by a Haddad militiaman, a man lost both legs when a five-foot hole was blown through the concrete wall. Two other Lebanese were killed by a mortar round as they rushed to help. They were Roger Ghannoum, a popular broadcaster at the Israel-based Voice of Hope radio station and a friend, Abdol Khouri.

Georgette Hasbani, 16, whose uncle, abraham Hobayka, based owns the house, described life in Dir Mimas as "an awful nightmare" during the weeks in which tensions between the militia and the PLO have built up. "I haven't slept in three nights.Every time we hear a bomb we run for shelter. What kind of life is this?"

The militias concentrated their fire on the coastal city of Tyre, Arnoun and Nabatiyeh, all PLO centers. Haddad's forces were also reported to have fired 155mm shells into a military barracks held by a leftists splinter group of Lebanese Army soldiers led by Lt. Ahmed Khatib, whose command issued a statement accusing Haddad and Israel of provoking the fighting.

Whether the fighting will escalate to the point where Israel will intervene remained a question today. The U.N. peacekeeping force appealed to all sides to restore the cease-fire that was arranged Aug. 26. The United States reportedly has urged Israel to exert a restraining influence on the militias.