About 50 U.N. peacekeeping soldiers and military observers entered southern Lebanon yesterday to begin direct international efforts to restore peace to the embattled area.

The force, which is to grow to about 800 to 1,000 soldiers from France, Iran and Sweden within the next few days, is to form a buffer zone to separate the invading Israeli soldiers from Palestinian guerrillas across the Litani River.

Israeli television reported last night that Israeli forces would be withdrawn from Lebanon "within the next few days," an indication that Israel believes the U.N. forces will be in place by then.

The U.N. force is scheduled eventually to reach 4,000 troops - about the same size as the force separating Israeli and Egyptian soldiers in the Sinai.

On this first day after the unilateral Israeli cease-fire, army officials here said the truce appeared to holding except for some isolated artillery and rocket fire that fell harmlessly in northern Israel. The Palestinians said the rockets were in retaliation for Israeli shelling.

Iranian soldiers made up the bulk of the initial troops that crossed the frontier from Israel. About 200 Iranians, transferred from another U.N. assignment on the nearby Golan Heights, are to guard the central sector of the front just south of the Litani River, including a key bridge.

A 250-man Swedish unit, expected the vicinity today, will patrol the eastern sector, while several hundred trench soldiers will man the key western sector stretching inland from the coastal city of Tyre which Israeli forces bypassed on their drive north.

In their one-week offensive, Israeli troops captured an area about the size of Rhode Island that represents about one-fifth of Lebanese territory.

U.N. officials reported good cooperation from the Israeli Army is moving the first contingents of what is called the United Nations Interim Force in Lebanon, or UNIFIL, through the battle-scarred border regions.

There was, however, trouble with the leader of the Christian militia forces in southern Lebanon, Maj. Saad Haddad, who said his men would stop U.N. forces from crossing the line. His forces refused entry to an observer team not connected with the new force.

The Christians, who have been fighting the Palestinians for years in the area, have accused U.N. observers in the past of spying on them and passing information on their activities to the guerrillas.

Nevertheless, the first elements of the new peacekeeping force that crossed into Lebanon from the northern Israeli town of Metulla met with no resistance from the Christians and was escorted by Israelis.

The Christian militia is a potential source of embarrassment for the Israelis in the present situation.

Because they are staunchly anti-Palestinian, the Israelis have been helping them for the past two years. Defense Minister Ezer Weizman this week referred to them as "our Christian brothers."

The Israelis have used the plight of the Christians in the region to stress their point that their eventual withdrawal from the south must be accompanied by guarantees that the Palestinian guerrillas will not be allowed to come back and terrorize either the Christian population or launch attacks against Israeli villages.

Yet, the frequently undisciplined Christian troops have also robbed and roughed up unarmed U.N. observers on many occasions in the past, some as recently as 10 days ago.

The planned U.N. force of 4,000 troops may not be much larger than what the Israelis have in the area now.

Although most unofficial outside estimates have placed the Israeli force at about 20,000, no official figure has been released and other foreign sources suggest it may really be closer to 5,000.

The Israelis hold roughly a 15-mile-deep belt across the 60 mile-wide Lebanese frontier. The Palestinians have been pushed back across the Litani where they occupy a 200-square-mile area south of the 40,000-man Arab peacekeeping force, mainly Syrians, brought in after the 1975-76 Lebanese civil war.

The Israelis have made it clear that they do not want to confront the Syrians, and the Syrians have also made it clear they are unwilling to be drawn into a fight with Israel. They key question, however, is whether the Syrians will take on a quiet role and try to support the cease-fire by restraining the Palestinians now on their side of the river.