Lebanese security forces moved into the town of Zahle today, ending a Syrian siege of Christian militiamen that had escalated into a full-scale Middle East crisis between Israel and Syria over the presence of Syrian missiles in central Lebanon.

About 100 militiamen of the Christian Phalange Party left Zahle under the watchful eye of Syrian forces who still control approaches to the city. Their departure was part of a decision taken last week at a special Arab League meeting called to seek ways of defusing the Syrian-Israeli standoff through such accommodation between Lebanon's warring factions.

It was not clear here today whether the Zahle withdrawal was part of an agreement that will lead to solution of the crisis over Syrian missiles deployed just down the road. But it nevertheless was regarded as a promising first step since Syria has cited the Phalange militia's presence in Zahle as justification for its attacks on the city.

It was the shooting down by Israeli jets in late April of two Syrian helicopters which the Israelis say were attempting to resupply forces on a mountaintop near Zahle that led to the Syrian display of SA6 and SA9 missiles the next day. This, in turn, prompted Israeli warnings that it would take action to remove the missiles if Syria did not.

The Israelis are supporters of Christian militiamen arrayed against the all-Syrian Arab Deterrent Force and Palestinian guerrillas in Lebanon. The confrontation over the missiles led to the shuttle missions of presidential envoy Philip C. Habib.

The ending of the long siege, during which Zahle's close to 100,000 residents took to basement bomb shelters to escape the near-constant Syrian artillery barrages, was arranged through mediation by the Saudi Arabian and Kuwaiti ambassadors here.

It was viewed by some Western diplomats here as a by-product of Habib's efforts to get the Saudis involved in his mission. Reportedly at Habib's urging, the Saudis persuaded the Phalange to pull back and the Syrians to allow them out.

One of Habib's goals, according to well-informed sources, was to reactivate joint Arab efforts to solve the Lebanese crisis. The plan reportedly was to defuse the crisis around Zahle and then seek an end to the Syrian-Israeli confrontation over the missiles.

While the Phalange pullout was watched by Syrian combat troops, some of whom wore the insignia of a skull and crossed swords marking a crack special forces unit, the militiamen were placed in the custody of Lebanese police officials.

About 350 special Lebanese police, wearing red berets, drove into Zahle at about eight this morning to bring the Phalange militiamen out.

Reportedly, all their heavy weapons had been turned over to the archbishop of Zahle, a predominantly Greek Catholic city just two miles down the road from this wine-growing center in Lebanon's lush Bekaa Valley that abuts Syria and is less than an hour's drive from Damascus.

Carrying their automatic rifles, the grim-faced Christian militiamen -- some hiding their faces from reporters -- rode out in five small gray buses with Lebanese police riding shotgun.

Before the militia was allowed to leave the staging area just outside of Zahle, the Syrian troops collected from inside the bedding that the Christian militia brought out heavy weapons including motors, machine guns, recoilless rifles and a 20-millimeter cannon.

The militiamen were taken in the buses to the Christian-dominated eastern sector of Beirut, where this afternoon the Phalange Party held at its war council headquarters a military parade of welcome complete with bands, guard of honor and medals.

Militia head Beshir Gemayel called the withdrawal of his forces "a moral victory" since Lebanese forces, not Syrians, have moved into Zahle. Joe Edde, commander of the Phalange forces in Zahle said his men did not want to leave despite the three months of pounding they had taken from the Syrians.

Edde said 200 civilians were killed and 2,000 wounded during the siege, but he refused to say how many of his fighters had died or were wounded.

The residents of Zahle reportedly greeted the Lebanese security forces with showers of flowers and rice -- a traditional welcome -- and celebrated the end of the siege with special church services and the pealing of church bells last night.

While the city was in Lebanese hands tonight, Syrian troops remained in control of the entrances and refused to allow any correspondents in. They cited the possibility of snipers and mined roads as the reason for their refusal.

The issue of Zahle has been viewed as the first step in settling Lebanon's six years of internal violence, including a 20-month civil war that ended in 1977 when Syrians, under an Arab League mandate, brought a tenuous peace to this politically and religiously splintered country.

The special Arab League committee that arranged today's withdrawal is scheduled to meet again on this country's problems Saturday in the Lebanese resort city of Beit ed-Din.