Eight hundred U.S. Marines in combat gear took up positions in the port of Beirut today to expedite the evacuation of Palestine Liberation Organization forces from the Lebanese capital.

The arrival of the U.S. 32nd Amphibious Marine unit came as the PLO withdrawal program so painstakingly negotiated by U.S. special envoy Philip C. Habib was set back by the postponement of the first planned overland evacuation to Syria because the mountainous roads to Damascus were still considered unsafe by the PLO.

The Marines landed at docks secured by French paratroopers last Saturday, when the sea evacuations were begun, as part of the buildup of a 2,130-member trinational peace-keeping force to oversee the two-week land and sea withdrawal of the Palestinians, who had been besieged in Moslem West Beirut for 10 weeks by the Israeli Army.

Within a short time, the Marines were monitoring the loading of vessels bound for Syrian ports with the forces that had been scheduled to move overland to Damascus.

As the Marine force bobbed ashore in a fleet of landing craft from the USS Guam helicopter carrier anchored beyond the port, there was a sense of deja vu for some early-waking Beirut residents, watching from high-rise apartments. Many remembered U.S. Marines landing in Beirut 24 years ago in 1958 to still a bloody civil war between the country's Christian and Moslem citizens.

Today's landing was a very different affair. In 1958 the Marines had arrived on a sandy beach south of a city still known as the "Paris of the East," wading ashore among laughing girls in bikinis and youths selling soft drinks.

Today they came into a desolate port of a city that has been savaged by its own fratricidal civil wars and by artillery shellings and aerial bombardments in an Israeli siege that has reduced West Beirut to a shattered ghost of its former modern splendor.

Instead of sand and beach cabanas, the Marines this morning were greeted by the somber sight of a devastated band of shell-broken buildings radiating from the port area to form the barren Green Line that cuts between the hostile eastern Christian and western Moslem halves of the capital.

There were no sounds of laughing, only the dull echoes of gunfire and exploding shells that are the ubiquitous cock's crow of every Beirut morning.

"It is certainly something different," said Roland "Ron" Lambert, a 46-year-old Marine warrant officer from Dover, N.H., who landed in Beirut not only in 1958 but had returned in June 1976 to help evacuate U.S. and other foreign nationals caught in the renewed civil war.

Lambert, dressed in combat fatigues and flak jacket, recalled that in 1976 it was the PLO, then the dominant force in the anarchy of West Beirut, that gave U.S. Marines protection as they evacuated the foreign civilians.

"They protected us then, and now we will protect them," he said. "I think it is great."

The Marines wasted little time preparing for another day in the chaotic sea evacuations that have been going on for the last five days.

Col. James Mead, a 6 1/2-foot Marine commander who was the first man to disembark, was greeted at dockside by Habib, the architect of the evacuation plan that brought a truce to the violent confrontation between Israel and the PLO. Col. Suheil Darghouth, the commander of the Lebanese Army units in the port area that are to join the multinational force in standing between Israeli and PLO forces around the city, saluted Col. Mead and welcomed the Marines back to Lebanon.

Within 1 1/2 hours after the first landing shortly after 5 a.m. Beirut time, three 210-man companies of Col. Mead's 2nd Battalion, 8th Regiment, 32nd Amphibious Marines were ashore along with a logistics force and were fanning out to take up positions separating Israelis and Christian Phalangist Party militias on the eastern side of the port from the Syrian-controlled Palestine Liberation Army at its western gates.

Col. Mead, 47, from Boston, reiterated that his Marines were there only to assure the peaceful evacuation.

"I am not anticipating any need for us to use our weapons," he told reporters, who watched as the Marines disembarked with M16s, heavy machine guns and the 60-mm mortars that are their heaviest weapons.

"We came here as peace-keepers. But obviously we would use whatever means we have at our disposal in the unlikely event that we had to defend ourselves," he said.

The first contact between the Marines and the heavily armed Palestinians, who dominated the shell-dissected Ottoman buildings of the once-quaint old business district of the city outside the port gates, tended to confirm that self-defense would be unlikely.

In an extraordinary encounter, given the rigid U.S. rules banning any direct contact between the U.S. officials and the PLO, Lt. Robert Johnston, 44, met this afternoon with Col. Salah Abu Zared, the commander of the Palestine Liberation Army units hunkered down outside the port.

The two chatted briefly about their respective deployments to facilitate the arrival of the truck convoys bringing Palestinian fighters to the port for evacuation.

When Marine Capt. Ken McCaid of Pleasantville, N.Y., complained during the meeting about the heat as he tugged at his heavy flak jacket, Col. Abu Zared laughingly said: "What are you worried about. It is perfectly safe here."

Col. Johnston, who said he was meeting the PLA commander as a member of an international peace-keeping force and not "exclusively" as a U.S. officer, said he had been "pleased" to meet his opposite number because his mission "represents the possibility of some peaceful settlement of the problem of West Beirut."

He said that the meeting "suggests that the prospects of peace might be in sight finally."

No sooner had he returned behind the sandbagged line of Lebanese Army officers separating his Marines from the PLA than there was a sudden crescendo of machine-gun fire, interrupted by the ear-splitting crack of recoilless cannons fired in salute of the first truck convoy of PLO evacuees of the day.

With Palestinian flags flying, the trucks moved up to the Marines amidst a bedlam of wild gunfire. Well-wishers tossed hand grenades into a nearby pile of rubble while others began blasting the skies with a deafening boom of rocket-propelled grenades.

The Marines, standing in a group before a bank building now being used as a sandbagged observation position, looked unfazed by the wild display of firepower, which has accompanied each departing group of evacuees through West Beirut over the past five days, killing as many as a dozen persons and wounding more than 100 from spent bullets falling back to earth.

The timetable for today's withdrawals had called for the evacuation of a group of some 1,000 PLO fighters by sea for North Yemen, while another 1,000 were to move overland to Syria. But the day's evacuation schedule was reshuffled at the last minute when the PLO obtained an agreement to have the overland movement canceled because promised security arrangements had not been made.

After an artillery clash between Syrian soldiers and Christian militia members from President-elect Bashir Gemayel's Phalangist Party in the Metn Mountains north of the strategic Beirut-to-Damascus highway yesterday afternoon, the PLO complained that the highway had not been secured, nor had the multinational force secured control of the city's two main crossing points between West and East Beirut that the PLO convoys would have had to use to reach the Damascus road.

PLO officials also said that the Israeli soldiers had not withdrawn from positions overlooking the highway or around the crossing points at the National Museum and Galerie Semaan as they were supposed to. Under the Habib plan, Israel was to pull back far enough from all evacuation points or routes so that they would not be seen by the departing PLO.

The U.S. Marine arrival today did allow for the initial French force of 350 paratroopers to leave the port area and begin to take up positions around the National Museum crossing point, which lies next to the shell-riddled residence of the French ambassador.

The arrival of another 450 French paratroopers by sea from Cyprus Thursday, as well as a boatload of 530 Italian soldiers who will take command of the Galerie Semaan crossing, is expected to remove at least one of the PLO's objections to the overland evacuation.

Meanwhile, the evacuation to North Yemen that had been planned for today was delayed. Instead, 560 soldiers of the Syrian military-controlled PLA, as well as a number of senior PLO officials, were trucked to the port to board the chartered Cypriot ferry Sol Georgios to sail to the Syrian port of Tartous.

The PLA units were part of the 3,500-member force that along with 1,500 Syrian Army troops had been in Beirut as part of the Arab League peace-keeping force sent to Lebanon to stop the 1975-76 civil war.

Among those leaving by boat for Syria today was Hani Hassan, Chairman Yasser Arafat's political adviser and a PLO Central Committee member.

A second ship, the Greek-registered Nereus, set sail from the port later this afternoon with about 600 of the 7,100 mainline PLO combatants who are being withdrawn along with the 5,000 PLA and regular Syrian Army units.

Today's departures brought to more than 3,500 those who have already left since the evacuation began last Saturday.

If security and logistical arrangements for the overland route cannot be completed in time, according to a clause in the evacuation plan, the whole evacuation would be made by sea.

Whatever the decision, it has become apparent that the initial schedules for departures are being modified almost daily now, giving rise to confusion about the exact numbers, destinations and departure dates of the evacuating Palestinian and Syrian fighters.

Meanwhile, scattered clashes were reported along the Green Line, mostly between remaining leftist militiamen in the west and members of Gemayel's rightist militia in the east, and the International Red Cross was preparing about 180 PLO wounded for evacuation Thursday on the 165-bed West German hospital ship Flora.