Israel, after a day of some of the heaviest bombardment of the war, said late today that its forces have accepted a new cease-fire in Lebanon, and U.S. special envoy Philip C. Habib quickly asked Lebanon's collapsing government to inform Palestinian guerrillas.

Although cease-fires have repeatedly been declared and broken in the nearly three-week-old Israeli invasion of Lebanon, reports of a new willingness by President Reagan to exert pressure on Israel and new political involvement by Saudi Arabia were seen here as indications that the ferocious fighting might be near an end.

The cease-fire came after a day of widespread bombing and naval shelling that reduced more sections of Palestinian and Moslem neighborhoods in West Beirut to rubble. It also followed the resignation of Lebanese Prime Minister Shafiq Wazzan, who expressed despair at the growing devastation and accused Habib of collusion with Israel in its use of "blackmail and military escalation" to stampede the Lebanese political establishment into abandoning the beleaguered Palestinian guerrillas.

The Israeli agreement to a new cease-fire reportedly came after Saudi Arabia, during the past two days, put strong pressure on the United States to use its influence as Israel's closest ally to halt the bombing and shelling.

The Saudi pressure reportedly included strong threats, whose exact nature could not be learned.

The cease-fire went into effect at 9 p.m. 3 p.m. EDT and there were no immediate indications of violations.

Throughout the day, however, Israel had continued to bombard the portion of the capital under Moslem and PLO control and tightened its hold on the strategic areas south and east of the city, centering on the Beirut-to-Damascus highway, the lifeline of the Syrian forces in Lebanon.

Israeli aircraft in pairs bombed and rocketed Beirut targets ranging from the much-punished Palestinian refugee camps in the southern suburbs to neighborhoods just south of Corniche Mazraa.

For Israel, that broad boulevard appears to be the dividing line between the Palestinian areas and the rest of West Beirut, which the invaders have bombed less intensely.

But the Israelis also renewed their naval gunfire along the entire coastline, from the closed Beirut Airport all the way up to the British Embassy, which was reported struck by one round, and the nearby American University of Beirut, where, according to university officials, several buildings were damaged for the first time in the current war.

Tentative casualty figures were given by police sources as 100 dead and 275 wounded for the day, after 30 were reported killed and 60 wounded in yesterday's raids, which had been the most intense since the first wave of the invasion on June 6.

Israeli tanks were reported in control of the Beirut Airport runways and main terminal building.

Military analysts ascribed greater strategic importance to Israeli ground moves east of Beirut. There, a column of American-made M60 tanks was seen moving up from Jamhour, just above the presidential palace at Baabda, toward Aley.

Syrian military spokesmen in Damascus said 1,000 Syrian troops had retreated from Alayh, a Druze town on the Beirut-to-Damascus highway, and taken up "another defensive line."

But military specialists said the Syrians, plus 400 Iranian and 200 Jordanian volunteers and perhaps 300 Palestinians, had been outflanked in a pocket south of the Damascus highway. They reportedly were not cut off, but their only escape lay in walking out through the Metn Valley north of the highway since no more roads existed for them to leave in their vehicles.

Wazzan, 56, charged in a state radio broadcast that while "Habib appears to be mediating to find a solution, he is giving Israel every possibility to go on with its aggression."

It appeared, according to sources here, however, that the key to the day's diplomatic developments was Saudi Arabia's decision, after weeks of silence, to intervene and use pressure.

Saeb Salam, a respected elder statesman who has been prime minister six times and took up some of Wazzan's activities today, told The Washington Post that PLO leader Yasser Arafat received word from Saudi Arabia that U.S. national security adviser William Clark had assured the Saudis that Israeli forces would make a token three-mile withdrawal.

If this version is correct--and highly placed Palestinian sources said it was--it was apparently on the basis of that assurance that Arafat expected that his own plan for a cease-fire would be accepted.

The PLO plan called for the guerrillas to return to their refugee camp bases and it virtually accepted the reduction of the PLO to the status of a political movement, not a fighting force.

Habib summarily dismissed the PLO plan, which had received the blessing of Lebanon's seven-member National Salvation Council set up to deal with both Israel and the Palestinians.

Yet even yesterday, after Habib's tough performance, PLO spokesman Mahmud Labadi insisted that the American special envoy was 24 hours behind developments in Washington. At that time Labadi did not explain, but apparently he was alluding to the Clark message sent through the Saudis.

The developments brought uncertainty here, then bitterness and charges of U.S. double dealing.

The Palestinian news agency Wafa reported later that the PLO had received important proposals about Beirut from the State Department Friday, Reuter reported. The agency said the PLO leadership was meeting to study the proposals and would send a reply to Washington Saturday.

Meanwhile the Syrian news agency Sana said that Arab foreign ministers will meet in Tunis Saturday to discuss Israel's invasion of Lebanon. The meeting originally was scheduled for next Tuesday. An Arab League spokesman in Tunis said most Arab countries are supporting the meeting.

The mood here was one of growing desperation even before Wazzan, a pro-Western lawyer, submitted his resignaton--and Druze leader Walid Jumblatt quit the National Salvation Council and ordered his two Druze ministers to resign from the government.

Salam said Arafat had been misled into believing that the United States had obtained Israeli assurances to pull back from the capital area to facilitate an "honorable surrender" of the guerrillas.

"Everyone seems lost," said Salam. The reaction of this traditional leader of the Sunni Moslem community was typical of a wide range of Lebanese political thinking on both sides of this divided capital.

Arafat was described by one Lebanese leader as tired and depressed and determined if at all possible to avoid going down in history as the political figure who sacrificed men, women and children in what increasingly appeared to be a futile fight against overwhelming Israeli firepower.

Arafat was quoted by the Palestinian news agency Wafa as telling his troops, "You are defending not only your lives but what little dignity and honor are left in this Arab world. You will determine the dignity of coming generations."

The resignations from the government and the National Salvation Council brought new concerns that there will be further fragmentation of what is left of the fabric of Lebanese society.

The now moribund council was the first attempt in more than seven years at persuading all major political and military factions in Lebanon to sit down at the same table.

Jumblatt, who had been close to the PLO leadership, told reporters in announcing his resignation that the council "now has been transformed into a council to bury the dead." He accused President Elias Sarkis, Wazzan and Foreign Minister Fuad Boutros of "plotting" by being more concerned with doing in the Palestinians than condemning the Israeli invasion.

Jumblatt said he was withdrawing from political life--thus apparently abandoning his title as leader of the 13 loosely associated leftist parties and militias known as the National Movement.

"I was trying to arrange an honorable surrender" for the PLO leadership, he said, "but I don't think the Israelis want them to leave Beirut. The Israelis just wanted them to stay and kill them, kill them, kill them."

"Let history judge," he said, warning that "Arab kings and presidents will reap the historical wrath of what has happened here." He predicted "the Arab world will be flooded in a sea of blood."

"None will be safe," he said. "I am predicting there will be a new PLO much more radical, much more extremist." graphics 1/photo: Rescue workers from Beirut's Civil Defense carried badly burned victim from a car bomb that killed six people in West Beirut Thursday night.