Five Greek passenger ships flying the U.N. flag sailed into Tripoli harbor early today to evacuate the besieged Palestine Liberation Organization forces of Yasser Arafat.

The ships' arrival from Cyprus had been delayed by Israeli gunboats which pounded PLO positions at the port yesterday, sinking one freighter and heavily damaging another.

The announcement yesterday that the ships and their escort of French warships had left Cyprus prompted a joyous celebation in the Palestinian-controlled section of this northern Lebanese city.

PLO guerrillas fired hundreds of rounds of Katyusha rockets, mortars and machine-gun tracer bullets just as they had when they left Beirut last year under a negotiated evacuation. The military commander of the mainstream fighting wing of the PLO, Khalil Wazir, also known as Abu Jihad, told reporters that Arafat and all 4,000 of his loyalist fighters who are surrounded here by Syrian-backed Palestinians opposed to Arafat would be evacuated today.

"We will be following the guarantees of the United Nations and of the other international parties," Abu Jihad said. "The Greeks sent their ships after they had all of the guarantees."

Following the announcement of the start of the evacuation operation yesterday, PLO trucks drove through Tripoli, with guerrillas raising victory flags, singing Palestinian nationalist songs and paying tribute to Arafat. Tripoli Mayor Achir Taye praised the PLO chairman, who presented the mayor with a certificate of appreciation.

Arafat told a cheering crowd in city hall, "I say to my brothers in Damascus, let's open a new page in relations. If we don't join forces we will be defeated."

The celebrations came just 11 hours after the most intense bombardment so far of the Tripoli port by Israeli naval ships in the Mediterranean.

As a result of the bombardment, the five Greek ships that had been scheduled to evacuate the guerrillas spent much of Monday anchored in Cyprus.

While the Israelis had not proclaimed a blockade of the Tripoli harbor, the effect of the bombardment--the second in 24 hours--was the same, as Greek maritime officials sought ways to ensure that the evacuation vessels would not be fired upon by Israeli gunboats.

Arafat said he wanted to proceed with the evacuation and had agreed to turn over the guerrillas' heavy weapons to Lebanese security forces as demanded by Israel.

One of the docked ships shelled yesterday, a Cypriot-registered coastal freighter, had figured in reports that the PLO was planning to remove its Katyusha rocket launchers, field artillery and antiaircraft guns before Arafat and his guerrillas set sail for Tunisia and North Yemen.

The ship, My Charm, which got direct hits on its superstructure, was anchored at almost the same spot where a hospital ship tied up last week to evacuate nearly 100 seriously wounded guerrillas. The freighter's crew was not aboard during the 30-minute predawn barrage.

A Lebanese security forces source said Arafat had been "very discreet" about moving the PLO heavy weapons closer to the dockside, and that he thought the Israelis merely had sought to ensure that they were not loaded onto freighters.

Israeli naval guns sank another docked freighter that had been gutted by gunfire in November during battles between Arafat loyalists and opponents.

Israel began shelling PLO positions in Tripoli after the terrorist bombing of a bus in Jerusalem Dec. 6 that killed six Israelis.

But its purpose in continuing to shell the port, thereby delaying the departure of the Greek ships from Cyprus, remained unclear.

There had been widespread speculation that Israel was trying to force Arafat to make a deal with Syria in which the 4,000 guerrillas would be evacuated overland through Syria, with Arafat having to yield considerable influence in the PLO to the Syrians in exchange.

Such a departure also would have denied Arafat the kind of dramatic withdrawal he and his guerrillas made from Beirut last year.

Some PLO officials said Israel merely wanted to draw attention to the terrorist aspect of the PLO as long as possible and humiliate Arafat for the purpose of domestic opinion in Israel.

Asked at a press conference why he thought the Israelis were preventing his evacuation, Arafat yesterday said, "You can ask them."

Lebanese Prime Minister Shafiq Wazzan criticized the Israeli shelling, saying that it obstructs the evacuation plans. He told reporters in Beirut that it is the responsibility of the United States to assist the evacuation by pressuring Israel. The evacuation is part of the overall program to rid Lebanon of foreign forces, he said, "and if Israel is serious about that goal, it should encourage all foreign forces to leave."

Israeli warplanes yesterday also attacked what the military command said were Palestinian terrorist bases in Syrian-controlled areas of Lebanon's central mountain range.

Israeli military spokesmen said the raids in the vicinity of Alayh and Bhamdun were intended to maintain pressure on Palestinian guerrilla bases. The Syrian-backed Druze Progressive Socialist Party's militia said the Israeli jets bombed a Palestinian Red Cresent hospital.

In the first major violation of a cease-fire renewal agreed upon Friday by Lebanon's warring factions, Druze militia forces and the Lebanese Army exchanged artillery fire for an hour yesterday in the hills southeast of Beirut. After a local truce was arranged, the Druze party declared that it considered the Friday cease-fire renewal in force.