A 16-man advance team of U.S. Marines helicoptered into the port of Beirut today, but the start of the key land phase of the Palestinian guerrilla exodus that the Marines will help guarantee was thrown into doubt later by an outbreak of fighting just north of the strategic Beirut-to-Damascus highway.

Confusion surrounded the fighting, reportedly an artillery battle, near three small Druze villages in the Metn Mountains 12 miles east of the capital.

Israeli radio claimed that the clashes were between Christian Phalangist Party militias of President-elect Bashir Gemayel and local leftist Moslem militias. That assessment was contradicted by the Phalangist radio, the Voice of Lebanon, which said tonight that the battle was between Israeli and Syrian troops.

Late today Israeli sources said that the overland evacuation, scheduled to begin Wednesday, had been canceled and apparently will be shifted to a sea exodus from Beirut port, Washington Post correspondent Edward Walsh reported from Jerusalem.

Earlier, Israeli military officials announced that the land exodus had been "postponed" at the instigation of Syrian authorities. The officials said they did not know the reason for the postponement or when the forces who had been scheduled to depart by bus for Damascus would leave the Lebanese capital. Details on A16.

[In Damascus, official sources said Syria has not stopped the evacuation and added that all measures have been taken to facilitate the Palestinians' entry into Syria, Agence France-Presse reported.]

The Marine advance team arrived today to facilitate a dawn landing Wednesday of the 800 other Marines who will join French and Italian troops in a 2,000-member international force as the naval phase of the PLO exodus, which continued without incident today, nears an end and the more complicated overland stage approaches.

The clash in the mountains east of this bruised and rubble-strewn capital came amid reports of new Israeli military movements and as new tensions gripped Beirut's Moslem western sector.

Travelers from southern Lebanon said Israeli forces were moving south toward the border Tuesday, noting that scores of tanks on transporters passed through the port city of Sidon for three hours, Reuter reported. In addition, Israeli tanks and armored personnel carriers have been seen moving out of East Beirut. The redeployment appeared to involve the whole length of the battlefront around West Beirut. Correspondents visiting the area around Beirut Airport, on the southern edge of the city, Monday night saw Israeli armored vehicles on the move, the news agency added.

The tensions in the capital followed a spate of leftist bombings to protest parliament's election yesterday of Gemayel, a 34-year-old rightist Christian, to be the next president of this war-splintered nation.

The selection of the controversial and forceful Gemayel by the 92-member National Assembly infuriated Moslem political leaders who had sought to boycott the vote to stop it. Leftist militiamen from the capital's ill-disciplined Mourabitoun, the major Moslem militia in West Beirut, responded to the election with a rampage of destruction of houses, offices and banks of those deputies that had defied the boycott and allowed Gemayel to be elected.

By midday today, the homes of six parliamentary deputies as well as that of Speaker Kamal Assad had been looted and set afire by dynamite bombs or rocket-propelled grenades, the street artillery of West Beirut's many private militias. Assad's office in the capital also was torched and two branches of the Beirut-Riyadh Bank that belongs to Hussein Mansour, another voting deputy, had been blown up.

Tonight, militiamen and looters returned to many of the bombed houses to strip them again and set them ablaze for a second time.

Editors at the newspaper An Nahar said that the homes of four other deputies in Lebanon's predominantly Sunni Moslem north, three in the port of Tripoli and one in the town of Khoura, had also been blown up. Armed clashes in the streets of Tripoli, Lebanon's second city, also had erupted between rival armed factions.

The new outburst of violence fueled concern here that the election of Gemayel, the youngest son of Pierre Gemayel, the founder of the nationalist Christian Phalangist Party, could set the stage for a new round of internecine violence.

Gemayel, who has warred with equal ruthlessness against his traditional Moslem antagonists as well as his fellow Christian political rivals, is anathema to the Moslem and Druze communities. They fear that with his election as president -- with the backing of the occupying forces of Israel, which have long supported his career and armed his 20,000-man strong militia -- he will now try to settle old scores from the 1975-76 civil war.

Having been outmaneuvered in parliament and weakened by the Israeli bombardments of their city that eventually neutralized their PLO ally, the Moslems have now found themselves suddenly in a vulnerable position with few political cards to play.

Because of that, the Moslem bloc led by former prime minister Saeb Salam was locked in meetings today seeking a united Moslem policy to deal with Gemayel's assumption of the presidency Sept. 23, on the expiration of President Elias Sarkis' six-year term.

A joint communique issued late last night after an emergency meeting of key Moslem and Druze leaders in West Beirut spoke of Gemayel's election as an imposition on the country "of a regime whose most obvious characteristics are a partisan spirit, dictatorship and fascism." Nevertheless, the door was left open to further negotiations with the Christian leadership in hopes of finding some political compromise between the two sides.

Although some Moslem leaders such as Druze chieftain Walid Jumblatt had demanded that Gemayel's election not be recognized because of alleged irregularities and intimidation of deputies, Salam appeared bent on finding a compromise with the Christian Maronite president-elect that would preserve the delicate Christian-Moslem balance prevailing since Lebanon's independence in 1943.

In a day full of political maneuverings, leftist street violence and reports of artillery battles in the mountains, the fourth day of the PLO's evacuation by sea from Beirut was hardly noted. But to the customary celebratory volley of gunfire, another truck convoy full of waving PLO guerrillas was taken through the city to the port to board a chartered Cypriot ferry, the Sol Express, for their trip to South Yemen, one of the eight Arab nations that have agreed to give them asylum.

With the port sealed off and secured by 350 French paratroopers and Foreign Legionnaires, and its outer perimeters patrolled by the Lebanese Army, the exact number departing today remained unclear. Various radio stations reported numbers ranging from 450 to 750. But a French diplomat who was at the port said that 570 had left today. That departure, the fourth shipload of PLO evacuees in as many days, brought to slightly over 2,500 the number who have left by sea since the evacuation plan mediated by U.S. special envoy Philip C. Habib began last Saturday.

In the course of the negotiations, the PLO had committed itself to withdraw 7,100 men. That would leave 4,600 to evacuate. Beyond that number, about 5,000 Syrian Army-controlled Palestinian and Syrian soldiers of the Arab League peace-keeping force were also to be evacuated in the coming days to Syria.

Sources involved with planning the overland evacuation said today that many details were still to be worked out. The original Habib plan provided only a general outline of the two-week evacuation, leaving specific schedules and means of departure to negotiations among Lebanese, Israeli, and officials and representatives of the three nations providing the international force.

One of the key problems remained how to secure the Beirut-to-Damascus road, which the convoys will have to take. Syrian forces are in many positions in the mountains north of the road while the Israelis dominate the southern part. Near Beirut the problem is complicated by the fact that the Phalangist militias of Gemayel, which are not part of the Habib agreement, are also in positions overlooking the road.

The Habib plan evisaged a discreet Israeli pullback from the road so that the departing guerrillas would not be humiliated by having to pass immediately in front of the Israeli Army.

PLO sources said tonight that many of the fighters were not eager to go by land out of Lebanon, fearing that either the vengeful Israelis, or the Phalangist militias who have always sworn to force all Palestinians from Lebanon, would attack them. The sources said that many of the fighters wanted to go to by sea, even though there are provisions for only two more ships to sail -- one Wednesday, with 1,000 guerrillas headed for North Yemen, and a second, which will go to Algiers, in the final days of the evacuation.

Thursday from 150 to 180 injured Palestinian fighters are to be evacuated from the port aboard the 162-bed West German hospital ship Flora, which was damaged in July by Palestinian rockets as it sailed from Juniyah, just north of Beirut.