The Israeli Army abruptly withdrew its armor-supported infantry from southern Lebanon today, four days after establishing three outposts there, and said its mission had been completed.

The withdrawal followed a cease-fire meeting by Israeli military commanders, officers of the U.N. peacekeeping force in Lebanon and Maj. Saad Haddad, head of the Christian militias who control a narrow enclave along the border with Israel.

Without ever saying what the purpose of its incursion was, the Army said all Israeli forces had left Lebanon and that talks with the United Nations and Haddad would continue later this week. Today's meeting was headed by Haddad, Israeli Maj. Gen. Yamosh Ben-Sal and Gen. Emanuel Erskine, commander of U.N. forces in Lebanon.

The withdrawal followed clashes between U.N. forces and Hadad's Israeli-supported militias yesterday, in which a Fijian U.N. soldier, a militia-man and two Lebanese civilians were killed. Haddad's forces opened fire on Irish and Fijian troops near the village of At Tiri, and the U.N. fforces responded with an American-made TOW antitank missile.

U.N. officials here said tonight that the situation in southern Lebanon remained tense, with Haddad's forces occupying part of the area that is supposed to be controlled by the U.N. Irish battalion.

Wednesday's incursion by the Israeli Army followed a Pallestine Liberation Organization guerrilla attack on the Misgav Am border kibbutz, in which three Israelis, including a two-year-old child, were killed. All five attackers were slain.

More than 300 Israeli troops dug into fortified positions along a 12-mile line stretching from the village of Marqabe to Kunin and appeared to be preparing for a long stay. Bulldozers formed earthworks surrounded by barbed wire and cut roads between the positions.

The response was restrained, however, compared to previous Israeli actions taken in retaliation to terrorist attacks. Usually such raids are followed by Air Force strikes and naval bombardments of PLO strongholds north of the Litani River, although incursions by troops normally have lasted only a few hours.

Reuter news service filed this report from Bint Jbail, Lebanon:

Several thousand residents of this Shiite Moslem village near the Israeli border vowed revenge on U.N. troops in Southern Lebanon at funeral ceremonies for the Lebanese militaiaman killed Saturday along with two Lebanese civilians and a Fijian U.N. soldier.

Four U.N. soldiers taken prisoner last night by the militia forces were exchanged for two captured militia-men. "The villagers wanted to kill them in revenge for the deaths of the youths but I managed to restrain them," said militia leader Haddad.

He charged that the two civilians were killed when Irish U.N. troops opened fire on a demonstration of about 50 At Tiri youths.

Haddad called the alleged Irish action "an act of complete savagery. From now on the Irish do not only have a military problem. They also have a civilian problem. I can no longer guarantee the safety of any U.N. personnel."

A U.N. spokesman at first denied that the Irish had fired at the civilian demonstrators, but later Maj. David Taylor, commander of the Irish unit at At Tiri, said his men opened fire under extreme provocation and the demonstrators were caught in the middle.

Taylor said the militiamen forced the youths at gunpoint to demonstrate and to throw stones and roll burning tires toward the Irish positions.

"At the same time militiamen opened fire with mortars and tanks on our headquarters, scoring at least two direct hits and killing the Fijian soldier. We returned fire and the demonstrators, caught in the middle, were hit."

He said the militias had been trying to take over At Tiri since last Monday.

[In New York, U.N. General Secretary Kurt Waldheim said during a Security Council meeting that Lebanese militiamen had fired first on the U.N. force, which he said had been subjected to "extreme difficulties and dangers" and had "exercised maximum restraint."]