The Palestine Liberation Organization distributed its heavy weapons and vast ammunition stores to leftist Lebanese militias today in apparent contravention of the U.S.-mediated agreement that the arms would be turned over to the Lebanese Army when the guerrillas withdrew from Beirut.

The move, which raised concerns about new confrontations in the war-weary Lebanese capital, came as more than 700 additional Palestinian fighters were evacuated by sea, bringing the total near the halfway point.

U.S. special envoy Philip C. Habib, who returned from Israel with a new agreement that is expected to permit the first overland withdrawal to start Friday, said the entire evacuation was ahead of schedule and might be completed before the original Sept. 2 deadline.

The PLO guerrillas today turned over tanks, heavy artillery, antiaircraft batteries, mobile rocket launchers, mortars, jeep-mounted recoilless rifles and huge supplies of shells and small-arms ammunition to the collection of leftist and Moslem militias that had fought at their side during the 10-week Israeli siege of West Beirut.

Under the agreement negotiated by Habib to end the siege and guarantee the PLO's peaceful evacuation of the city, Palestinians were allowed to depart with their small arms--assault rifles and pistols--but were to turn over all their heavy weaponry to the Lebanese Army.

The PLO has refused to do this because it and its Moslem allies consider the Lebanese Army as an instrument of the right-wing Christian Phalangist Party, which has been the Moslems' most implacable enemy in Lebanon. The Moslems remain suspicious of the Lebanese Army because Christian officers dominate the upper echelons of its command structure.

The PLO action appears to set the stage for an eventual showdown over the arms between the leftist militias and Bashir Gemayel, the Phalange militia chieftain who was elected president this week and is to take office Sept. 23.

The heavy weapons were distributed to Nasserist, socialist, Shiite Moslem and communist militias who remain in their rubble-strewn positions around West Beirut as PLO Chairman Yasser Arafat made the rounds of the city that has been his home for 12 years to say his final goodbyes to colleagues and Moslem leaders who have been his allies.

Amid crowds of teary-eyed supporters, Arafat stopped at a number of his PLO offices in the heavily bombed Fakhani district, embracing comrades and office workers. He held an emotional session with his brother, Fathi, the head of the Palestine Red Cresent, who left Beirut this afternoon with 165 wounded PLO fighters on board the German Red Cross hospital ship, Flora.

For security reasons, Arafat has kept secret the exact date and method of his departure. But his melancholy pilgrimage among those who had fought beside him and supported him was taken as a sign that his departure was imminent.

Late last night he drove to the West Beirut home of former prime minister Saeb Salam, the man who negotiated for the PLO with Habib. They met for an hour in private and before a final embrace, Arafat gave Salam, whom he called "our big brother," his ebony swagger stick.

Salam said today that Arafat would not leave by any of the overland convoys that will begin to depart for Syria Friday along the Beirut-to-Damascus road. Arafat, Salam said, had been given U.S. and French guarantees of safe passage by sea and would thus be leaving from the port of Beirut, which has been patrolled since Wednesday by 800 U.S. Marines.

The Marines, who landed from a seven-ship flotilla of the 6th Fleet that was still standing off the port today, are part of the 2,130-man international force to oversee the PLO withdrawal set up in the Habib plan that was approved by Israel, Lebanon and the PLO leadership.

The multinational force was brought to full strength today with the arrival of an additional 450 French paratroopers and 530 Italian troops, who came wearing their traditional plumed hats and with what their commander, Lt. Col. Bruno Tosetti, said were "truckloads of pasta" that would make their unit "completely self-sufficient."

The U.S. Marines continued to hold the port, from where a shipload of 697 PLO fighters set sail today for North Yemen, along with the Flora that set out with its wounded for Cyprus and Greece.

The French and Italian troops deployed around the two important crossing points along the battered Green Line dividing the Christian eastern and Moslem western halves of the city, along the route which the first overland evacuation to Damascus is to begin at dawn.

The evacuation on the overland route that is to carry most of the remaining 7,100 PLO fighters and 5,000 Syrian and Palestinian soldiers of the Syrian peace-keeping force that has been in Lebanon since 1976 was to have begun yesterday. But it was postponed when the PLO complained that there was insufficient protection for its convoys passing through mountains where both Phalangist militias and Israelis hold positions.

More than 4,500 Palestinians now have left Beirut with another 1,300 scheduled to leave Friday.

Israeli Defense Minister Ariel Sharon said Thursday in New York that Israeli diplomats are planning a peace treaty with Lebanon that will probably be presented to the new Lebanese government and ratified by both governments "very soon." United Press International reported.

Habib left his headquarters at the home of the U.S. ambassador here and flew to Jerusalem yesterday to work out the final security details to get the overland evacuation started.

It is understood that the Israelis and their Phalangist Party allies committed themselves to pulling back from the road enough to be out of eyesight of the passing PLO convoys. The convoys will leave through the Italian-held Galerie Semaan crossing point and proceed along the road escorted by a token force of French paratroopers who will accompany them to Sofar in the Bekaa Valley, where Syrian lines begin.

The arrangement made by Habib, who returned to Lebanon today, allowed Syria to send a fleet of 60 trucks to West Beirut today to begin loading the 1,300 men of the Syrian-controlled Palestine Liberation Army who will be the first to leave Beirut by land.

They are to be followed by daily convoys of other Syrian Army, PLA and PLO guerrillas until all Palestinian and Syrian combatants have been evacuated. Habib, who was clearly relieved to have arranged the overland passage out of Beirut, predicted today that the evacuation could be completed ahead of its scheduled Sept. 2 deadline.

All afternoon, amid the makeshift barracks along the skeletal remains of shattered buildings behind their positions on the Green Line, PLA members could be seen loading up their duffel bags, ammunition boxes and newly bought Japanese radios, into the trucks that are to take them to Syria shortly after dawn.

At nightfall tonight, as their final campfires glimmered in the eerie dark that was illuminated at times by tracer bullets loosed into the sky, tank-backed leftist militiamen, mostly of the Nasserist Mourabitoun, waited in the shadows to occupy the PLA position the minute they were abandoned by the evacuees.

Armed with the sort of fire power they had only dreamed of before, the Mourabitoun militiamen were telling visitors that for them, the war was not over. If the Christians try to move into West Beirut, or if they send the Lebanese Army to try to disarm them, they said, they would fight, just as they had in the 1975-76 civil war from some of these same positions and just as they had this summer when they fought alongside the PLO to hold off Israeli attacks on the city.

"The Palestinian problem is perhaps finished in Lebanon," said one bearded Mourabitoun commando today, refusing to give his name. "But the Lebanese problem is not."

The Mourabitoun forces said they would fight to oppose the coming of "fascism" to Lebanon, as they termed the election of Gemayel.

"This fascism, the Phalangists who are supported by the Israeli occupiers, will not enter West Beirut," one said. "We will never forget how many of our women and children were felled in this war because of them -- the Israelis and the fascists."