The last major Arab obstacle blocking evacuation of the besieged Palestinian guerrillas from West Beirut was removed today, and a key negotiator confidently predicted that their departure could begin "by the middle" of next week.

Former Lebanese prime minister Saeb Salam told reporters that Syrian President Hafez Assad finally agreed today to accept the guerrillas after the Palestine Liberation Organization formally asked him to do so in writing.

Another mediator, Lebanese Prime Minister Shafiq Wazzan, said in a nationally televised statement after meeting with U.S. special envoy Philip C. Habib, "I am cautiously optimistic that the evacuation will begin within a few days." He described Habib as "completely satisfied with the positive responses" from the PLO to clarifications Habib had sought on Palestinian positions.

Habib, in keeping with past practice, did not comment on the negotiations.

Israeli officials, who said Habib had forwarded to them a PLO withdrawal proposal, acknowledged there was a "glimmer of hope" for a peaceful resolution of the crisis, Washington Post correspondent Edward Walsh reported from Jerusalem.

But the officials stressed that many key points remain to be hammered out before a settlement could be reached, notably whether the PLO would meet the Israeli demand that the guerrillas begin to leave Beirut before the arrival of an international peace-keeping force. One senior Israeli Foreign Ministry official also questioned whether the PLO was sincere in offering to withdraw.

In Washington, there was no official comment, but sources said Habib was expressing cautious optimism that the negotiations were progressing to a successful conclusion.

Despite reports of sporadic exchanges of gunfire between Israeli and PLO snipers during the day, the Lebanese capital enjoyed its first full day of relative calm in a week that was marked by the heaviest Israeli bombing and shelling attacks of the two-month conflict.

But the atmosphere in the besieged city remained one of extreme anxiety. Unconfirmed reports circulated that Israeli forces were poised to strike again either Sunday or Tuesday. Palestinian officials noted that Habib yesterday had promised to put the finishing touches on his evacuation agreement within five days and voiced worry that the Israelis would strike during the delay.

Jean-Jacques Kurtz, the International Red Cross spokesman in West Beirut, warned that if the city underwent another day of shelling, "there will not be a single bed in any hospital in West Beirut for wounded people." At least nine of the sector's 15 hospitals and 27 clinics were hit in last Wednesday's 20-hour bombardment and several others were damaged in earlier attacks.

Even an Israeli decision authorizing the International Committee of the Red Cross to bring in one truck of medicine and four filled with food--including infant formula--failed to relieve the sense of helplessness here.

Residents of West Beirut wandered aimlessly through largely empty streets. Thousands more fled to the relative safety of the east through the one checkpoint the Israelis have kept open. Relief workers reported thousands of refugees without money or homes in the hills above the capital.

Israel launched a 15-minute artillery barrage on West Beirut at dusk, but otherwise there was little fighting, The Associated Press reported. The Israelis reported one soldier wounded when his vehicle hit a mine near Tyre, south of Beirut.

Rescue workers were still digging today through the rubble of a six-story apartment building housing a PLO operations center that was demolished yesterday in an Israeli air raid. Many civilians also lived in the building. So far more than 20 bodies have been recovered from the wreckage and 22 injured survivors taken to the American University of Beirut Hospital.

Palestinian officials, and the Soviet news agency Tass in a dispatch from Beirut, charged that Israel had used a new precision weapon to flatten the apartment building. Tass said it was the first time in military history that the weapon had been used operationally.

With the predominantly Moslem western sector ringed by about 300 Israeli heavy artillery pieces, 500 tanks and dozens of gunboats offshore, the Palestinians offered several indications that they wanted to conclude an evacuation agreement as quickly as possible. If Israel agreed, the 15-day evacuation plan now under discussion was expected to start promptly.

The PLO has jettisoned one condition after another and was reported to have issued its guerrillas new uniforms to make a good impression when they are evacuated.

Racing to head off any further Israeli assaults, Habib scheduled for Sunday a meeting of American, French and Lebanese Army officers to iron out the final evacuation timetable. The American officer who will take part in those talks was reported to have arrived today.

French diplomats reported that if an agreement was reached, their troops--either the 800 paratroopers on United Nations duty in southern Lebanon or a unit based in France--would be available. The paratroopers could arrive in Beirut within 24 hours, the diplomats said.

Italian Premier Giovanni Spadolini, in one of his last acts before resigning, offered Italian forces for the peace-keeping mission.

In Athens, Greek government sources said following a meeting between Prime Minister Andreas Papandreou and PLO political aide Farouk Kaddoumi that Greece was ready to dispatch 300 peace-keeping troops to the Lebanese capital.

The PLO appeared to have backed away from its previous insistence that an international peace-keeping force be deployed before the evacuation of its fighters begins. Officials suggested that the PLO could begin withdrawing a first group of wounded guerrillas and about 1,000 foreign volunteers on the day before the French vanguard of the international force takes up positions in West Beirut.

The PLO also promised to provide Habib with exact numbers of guerrillas to be evacuated. Past estimates have ranged from 5,000 to 9,000, but Salam said today there might be "a bit more."

But despite the PLO concession, the timetable question remained unresolved. Salam and the Lebanese Moslems he speaks for remained adamant that the international force's arrival had to be "simultaneous" with the departure of the first Palestinians.

Israel has insisted repeatedly that the Palestinians depart before the arrival of the international force, which is reported to consist of American, French, Greek and Italian units.

Another apparently unresolved matter was the question of where all of the guerrillas would go. While Jordan, Syria and Iraq all reportedly have agreed to accept some of the Palestinians, Egypt denied today that it also had consented to take in a portion of the guerrillas, although Salam claimed that the Lebanese government had been assured of Cairo's cooperation.

Egyptian Foreign Minister Kamal Hassan Ali said in Cairo that Egypt had not been informed officially that the PLO is ready to withdraw from Beirut. He also insisted that a PLO withdrawal "must be preceded by the establishment of a global settlement of the Palestinian problem." Israel has insisted that there be no linkage between the withdrawal and other Palestinian issues.

Without Egypt's cooperation, about 1,500 of the trapped PLO fighters could conceivably be left without a place to withdraw to.

There also remains the question of how to define who is a guerrilla. The PLO has insisted that many of its forces consist of civilians who serve as part-time soldiers and who should not be forced to withdraw with the rest of the PLO contingent.

Previous optimism about an emerging agreement between the Lebanese, Palestinians and Habib was shattered by this week's Israeli assaults.

Just one week ago, Habib had differed sharply with French Ambassador Paul Marc Henry, who expressed doubts to Lebanese President Elias Sarkis about Israeli willingness to abide by the cease-fire that the American envoy had worked out the previous night, informed sources said. These sources reported that Habib said the honor of the United States was at stake and swore that the cease-fire would be honored.

When the Israelis unleashed a 14-hour assault on Sunday, Habib was so shaken that the next day he called on Sarkis and said he would understand if the Lebanese government no longer had confidence in his mission, the sources said.

There were dozens of sometimes conflicting accounts concerning details of the PLO withdrawal plan. But there was general agreement that the PLO had agreed to surrender its heavy weapons to the Lebanese Army at the end of the evacuation process and that many PLO forces would leave under a U.S. guarantee of safe passage rather than insist on an international escort for protection from the Israelis.