Syrians declared a unilateral cease-fire in Lebanon last night after a week of ferocious fighting with Christian militias. A precarious calm settled over Beirut's battlegrounds.

Christian leaders had said earlier they would respect the U.N. Security Council's call for a cease-fire if Syria did. The official Beirut radio announced that the mainly Syrian Arab League peacekeeping force would stop shooting at 8 p.m. and as far as could be determined it did.

Although there was no way to predict how long it would hold, the cease-fire brought at least a temporary halt to more than a week of artillery barrages that observers said surpassed even the 1975-76 civil war in ferocity.

The last serious incident before the truce was an artillery barrage against Beirut International Airport. Around nightfall six rounds fell in our around the Syrian-controlled airport in southern Beirut with one shell landing near an incoming flight from London.

The Middle East Airlines Boeing 707 was taxiing into the terminal at the time. No one was hurt but passengers had to rush out of the airport leaving their luggage on board the plane.

The source of the shooting was not immediately clear. During the civil war Christian gunners closed down the airport for months at a time by shelling and rocketing it from afar.

The cease-fire resulted not only from the Security Council appeal but also from consultations in Damascus between Syrian President Hafez Assad and Lebanese President Elias Sarkis. Assad cut short a visit to Moscow to return to Damascus and within hours Sarkis was conferring with him.

However, public confidence in the cease-fire was extremely limited, especially since its terms were not immediately made public.

If the Syrians remained in their key strongpoints in East Beirut - atop an unfinished skyscrapers and across two key bridges - the Christians would be no closer to their stated goal of driving the Syrians out of Lebanon than they were before the latest round of violence exploded.

What effect the fighting would have on Christian efforts to prevent renewal of the mandate of the Arab force - which expires Oct. 26 - remains to be seen. The large-scale destruction wrought on Christian neighborhoods, most all but deserted by their residents, has cast a shadow over Syrian's claims of being impartial peacekeeping troops.

An Israeli gunboat raid Thursday night, interpreted as a warning to the Syrians on behalf of Israel's Christian allies, had no immediate effect on the tempo of the Syrian heavy artillery bombardment.

It was only after the Security Council appeal, joined by Syria's influential Soviet allies, that Damascus made the decision to silence its big guns.

But there was no way to gauge whether the Israeli warning and the wish to avoid a Syrian-Israeli confrontation played a role in the Soviet decision to back the Western efforts that led to the Security Council truce call.

The Israelis maintained officially that their raid was aimed at guerrilla nests among Palestinian refugees who have sought shelter in Beirut beach cabanas. But political observers had said it could be a prelude to tougher and more extensive intervention if the Syrians continued their onslaught.

In a reflection of these fears, the Lebanese National News Agency yesterday printed a list of procedures for the public to follow in case of an air raid.

The syrian troops, backed by Soviet-made T54, T55 and T62 tanks, number approximately 30,000. They are the iron fist of the Arab Deterrent Force set up by the Arab League and assigned in 1976 to keep the peace among Lebanon's feuding Christians, Moslems and Palestinian guerrillas. Others in the force include Saudis and Sudanese, but they have not been involved in the latest fighting.

The current round of fighting erupted shortly after the Camp David accords were announced, leading to speculation by some observers that Assad deliberately sought to raise tensions in the Middle East to undermine the agreements between Egypt and Israel.

Others pointed out, however, that the Christians were equally disappointed by the Camp David framework for the future of the West bank and Gaza. No provision was made, they complained, for return of the approximately 400,000 Palestinians in Lebanon, who the Christians maintain are the cause of Lebanon's affictions.

In addition, the simple presence of Syrian soldiers and the heavily armed Christian militias - particularly during a dispute over renewal of the Syrian's mandate - set the stage for confrontation that could easily escalate into all-out battles.

Whatever its immediate cause, the fighting did considerable damage to the predominantly Christian Ashrafiyeh quarter, driving out thousands of Christian families and killing more than 500 persons, mostly civilians, by conservative estimates. Christian sources put the toll as high as 900.

The fighting had continued intensely before last night's cease-fire call. The Voice of Lebanon, radio of the rightist Christian Phalange Party, said Christian gunmen again assaulted Syrian positions on the Karantina and Beirut River bridges.

The two bridges command the main coastal highway leading from Ashrafiyeh, which is partially cut off, northward up the seaside toward Jounieh, the Christian port where most arms and munitions for the militias arrive.

Boats leaving from Jounieh also are carrying refugees away from the war zones to the nearby Mediterranean island of Cyprus. The seaside route also leads into the Mount Lebanon area, where many of Ashrafiyeh's residents have fled to live with relatives in their home villages or in weekend villas.