Israel unleashed a massive series of air, artillery and sea attacks on Palestinian positions in and around West Beirut today, shattering a relative calm that had lasted three days and raising new fears that a U.S. plan for a peaceful resolution to the Lebanon conflict may come unstuck.

The Israeli attacks came as U.S. special envoy Philip C. Habib briefed Lebanese President Elias Sarkis on his proposal for the evacuation of Palestinian guerrillas from the besieged capital. Both men awaited an Israeli reply that could set the evacuation in motion as early as the end of the week.

An official reply to the proposals that Habib passed on to Israeli Defense Minister Ariel Sharon in a tense, three-hour meeting yesterday was expected from Jerusalem sometime Tuesday, possibly after a further meeting between Habib and Sharon.

As the American negotiator met with Sarkis and with Lebanese Prime Minister Shafiq Wazzan, Israeli jets launched a series of attacks on Palestine Liberation Organization artillery positions located behind Syrian Army lines in the mountains about 19 miles east of the city. Almost simultaneously, Israeli tanks and artillery pounded Palestinian defenses near Beirut's National Museum, one of the key crossing points between the divided parts of the city.

Artillery duels were continuing early Tuesday morning between Israeli forces and Palestinian guerrillas, Reuter reported.

A military spokesman for the Israelis claimed that their forces had made a slight advance down the heavily defended road beyond the museum, Washington Post correspondent Leon Dash reported from East Beirut.

"Things are very hot in the museum area and it's a reinforcing movement," the spokesman, Yehiel Ben Zvi, told reporters. "If there is going to be a major push soon, if I knew, I wouldn't tell you . . . . The day we get the order to take West Beirut , we'll move in."

The attacks intensified in the afternoon, as Israeli jets staged repeated bombing and rocket assaults for three hours around the city, while Navy gunboats pummeled the once-fashionable apartment and embassy quarters along the southern seashore. Throughout the day, Israeli artillery fire rained down on the battered Palestinian refugee camps of Shatila, Sabra and Burj al Barajinah on the city's southern outskirts.

Habib, who for two months has sought to mediate a peaceful end to the conflict, was reported to be alarmed by the renewed Israeli attacks.

An hour after Israeli jets began to drop bombs around the city this afternoon, Habib telephoned former Lebanese prime minister Saeb Salam, who has acted as his intermediary with PLO leader Yasser Arafat. Salam said Habib urged him to contact the PLO leader to advise restraint in responding to the raids.

Salam said he tried to reach Arafat and his aides by telephone but got no answer, an indication that the PLO leaders probably had already gone on full combat alert and abandoned their regular offices.

"We are in a race between peace and destruction," Salam said as the roar of jets over his office was followed by the distinct popping of Palestinian antiaircraft fire. "This is a race between Habib and Sharon."

The PLO's official newspaper, Falastin Ath Thwara, yesterday spelled out in an editorial the position that Arafat has already communicated to Habib through Salam. "We have taken the decision for a military withdrawal from Beirut," the editorial said, "because the destruction of Beirut over the heads of half a million Moslems is not a mere probability but has become a reality."

With Israeli tanks and troops reported to be massing to the north, near the port of Beirut and at the museum crossing, and with other Israeli units being reinforced in strength just south of Beirut International Airport, there was growing concern here tonight among diplomats and Lebanese officials that Sharon might be planning what one European ambassador here described as "one last thunderbolt" against the city.

"This is the man who believes in the fait accompli," Salam said of Sharon. "And Israel itself is a nation that believes in the virtue of creating its own facts."

No one here doubts that Sharon would prefer a military solution in Beirut which would decapitate the PLO leadership, and leave the organization as weakened politically as it has been militarily by the Israeli invasion that began June 6.

Lebanese officials expressed concern over an Israeli radio report, citing the Christian Voice of Lebanon, that troops under the command of former Lebanese major Saad Haddad, an Israeli ally, had moved to positions along the capital's southern perimeter. They said the possible use of Haddad's Christian troops in an assault on predominantly Moslem West Beirut would exacerbate the already-tense divisions between Christians and Moslems in this war-torn country.

"They say they want a strong Lebanese government," a Lebanese businessman said, "but the Israelis are doing everything in their considerable power to ensure that they hold sway in the areas near the border and to hell with the rest of the country."

Concern was also expressed here over the recall of Israel's ambassador to Washington, Moshe Arens, to Jerusalem for consultations with Prime Minister Menachem Begin. There was speculation here that Arens was being sounded out on just how severe the reaction would be in Washington if the Habib proposals were either rejected or if an answer were postponed to give Sharon a final chance to attack the PLO.

While Israeli officials have continued to maintain publicly that they are still waiting for firm signs that the PLO is prepared to leave Beirut and that Arab countries will take them in when they do, sources close to Habib say all these matters have been satisfactorily resolved and communicated to the Israelis. One Lebanese politician also said that the PLO, in effect, has given Habib a blank check to accept any Israeli conditions he decides are reasonable.

Despite the uncertainty today, logistical plans for the PLO evacuation and deployment of an international military force to take over their positions were completed in a meeting at the Lebanese Defense Ministry of military representatives from the United States, France, Italy and Lebanon.

The plans, which were passed on to Habib for relay to the Israelis, were reported to provide for a 14-day period for the evacuation, which would begin the same day as the arrival of the first contingents of the international force, expected to be a 350-man paratrooper unit from France. French troop transports are already standing off the coast of Cyprus, and French diplomatic sources say the force would be airlifted to Cyprus and then transferred to the ships that would sail to Beirut.

This initial French contingent would arrive on the same day that the first PLO guerrillas depart by sea for the Jordanian port of Aqaba. The rest of the evacuees would be moved overland to Syria along the Beirut-Damascus road in later stages of evacuation.

Lebanese and PLO officials here say that, contrary to Israeli statements, commitments have been received from Syria, Jordan and Iraq to receive the Palestinian guerrillas. There are still hopes that Egypt also will take some.

As the PLO units abandon their positions in and around Beirut, further units of the international force would take their place. The international force is expected to be made up of 2,000 soldiers from Italy, the United States and France.

Implementation of the plan awaits assent from Jerusalem, according to Salam. If the plan were to be approved Tuesday, as hoped, Salam said the evacuation could start as early as Saturday.

But the dull thud of exploding artillery echoing throughout the night did not encourage optimism for such an early solution. "I fear that before we have an agreement," said one PLO spokesman tonight, "we are going to have more violence."

News services reported these developments:

A crowd of about 5,000 Palestinian demonstrators in Damascus, Syria, broke into the U.S. Embassy compound, tore down the American flag and replaced it with a Palestinian banner Monday to protest U.S. support for Israel.

The protesters, who also stormed the Jordanian Embassy, smashed windows and set up huge portraits of Arafat on the building.

In an interview published in Le Monde in Paris, Arafat said the PLO had reached a final agreement with Lebanon and the United States on withdrawal of its forces. But he said the guerrillas would fight to the death if attacked by the Israelis before the evacuation took place.