Thousands of frightened residents of West Beirut today fled their besieged sector of the capital despite relative quiet and optimism prompted by new Palestinian concessions designed to achieve the guerrillas' negotiated departure with American guarantees.

Giant traffic jams quickly built up after the Israelis opened a single crossing point--the southernmost checkpoint known as the Galerie Semaan--a day after they subjected West Beirut to the harshest punishment yet in their now two-month-old invasion. Police officials said 250 civilians were killed and 670 wounded in what appeared to reporters to be indiscriminate Israeli shelling of both Palestinian strongholds south of the city and major neighborhoods in West Beirut itself.

On the diplomatic front, Lebanese politicians and Palestinian officials both waxed optimistic about drafted proposals that could lead to a breakthrough in efforts by U.S. special envoy Philip C. Habib to obtain the withdrawal of the Palestine Liberation Organization from Beirut.

The proposals offered not only American guarantees for the physical safety of departing guerrillas but also an apparently open-ended commitment to protect Palestinian civilians in the future when they will be without a PLO presence, according to these officials.

A major go-between in the negotiations, former prime minister Saeb Salam, told correspondents that the PLO had proposed a timetable for its withdrawal over two weeks, a significant change from the one-month period that the Palestinians had sought earlier.

The proposal was included in a five-page document that Salam said was the latest, detailed Palestinian proposal worked out in tortuous, indirect exchanges with Habib late Tuesday night just as Israel was launching its latest strike against West Beirut.

Salam confirmed that the PLO had also dropped its demand that a multinational buffer force be put in place before a guerrilla withdrawal could begin. The Palestinians now say that they will begin to withdraw simultaneously with deployment of a multinational force.

The Israelis encouraged today's exodus from the western sector of the city, apparently in hope of depriving the Palestinians of what they have denounced as a "screen" of Lebanese civilians.

Only sporadic small arms and artillery exchanges marred the day--and a series of mock, low-level air raids, which caused panic at dusk--but the half million residents of West Beirut understood the message. Increasingly, even people who remained now abandoned the predominantly Moslem sector's peripheries and crowded into the most central neighborhoods.

Near the crossing point, professional men and their families, who for weeks had refused to abandon their homes to squatters and thieves, waited for hours in the sweltering heat. They were joined there by truckloads from the threatened suburbs of poor Shiite Moslems, the women recognizable by their kerchiefs covering their heads.

Militarily, both the Israelis and Palestinians consolidated defenses as specialists assessed the Israelis' massive attack yesterday. They found it at best a half victory, which had cost the Israelis dearly in casualties.

By Israeli standards, the deaths of 19 soldiers was heavy and signaled that beating the guerrillas militarily could prove a vastly more painful operation than casualty-conscious Israel may be prepared to suffer.

Either by design or because of stubborn resistance from the Palestinians and their Lebanese allies, the Israelis only managed to maintain a thin toehold in the woods south of the museum checkpoint where their armor invaded just after midnight yesterday, eyewitnesses said.

Determined Palestinian resistance limited the Israeli advance around the 30-acre racetrack adjacent to the museum. Using shoulder-fired, rocket-propelled grenade launchers, the guerrillas appeared to have stopped the Israelis from advancing down the main avenue leading from the racetrack. It was unclear if fighting continued in the racetrack or in the woods surrounding it.

There was no new movement in the port today, where yesterday the Israelis had made an initial advance but then retreated to previous positions in the middle of its sprawling perimeter.

The Israelis had not achieved a definitive victory even in the south, where their armor thrust northward from the airport was now reported poised near the Kuwaiti Embassy, which dominates two major Palestinian refugee camps. It was unclear whether the Israelis had succeeded in isolating any parts of the camps, now largely deserted by civilians but still controlled by PLO fighters.

Barring a sudden collapse in now buoyant Palestinian morale, the Israelis appeared far from their apparent original goal of slicing south from the port and west from the museum to link up with the more successful armor column.

Underlying the optimistic mood surrounding the negotiations was an unspoken awareness that PLO Chairman Yasser Arafat has abandoned many of his previous conditions and genuinely seems determined to leave if only Israel will let the guerrillas do so.

Habib himself was said by both Palestinian and Lebanese insiders to be pleased with the progress of the negotiations.

But such optimism was tempered by fears that the United States could not ensure that Israel would stop escalating its military pressure, which the sources feel is designed to destroy the PLO.

Analysts suggested that Salam's uncharacteristically open bit of diplomacy in publicly describing the PLO's position was aimed at putting both Lebanese and Palestinians on record as favoring a negotiated settlement and thus forcing Israel to stop accusing the PLO of foot-dragging.

"I think there is going to be a political solution," Salam said, despite the many dashed expectations that have led to caution over the weeks. He said that he had told Habib that if principal obstacles could be overcome, then "I take it upon myself to make the Palestinians accept everything" in the envoy's most recent proposals.

Speaking for the PLO, Bassam Abu Sherif made similarly conciliatory promises.

A potential sticking point, according to a Palestinian source, was the proposal's provision that the departing guerrillas should be allowed to keep their heavy weapons as well as the individual weapons that the Israelis have accepted. However, the source made clear that the PLO was prepared to drop that demand if necessary.

Even were Israel to accept the proposal in principle, Salam and Palestinian officials made clear that other crucial details would remain to be worked out.

For example, the exact number of guerrillas to leave, which on paper Israel demands should be as large as possible and the PLO just the opposite, must be ironed out. So too must be the vexing problem of how many men would go to each of the known recipient countries, Egypt, Iraq, Jordan and Syria.

The Palestinians have asked that French troops be deployed as the first elements of the multinational force and now apparently would begin their withdrawal simultaneous with the French deployment. PLO negotiator Hanni Hassan said that the French government has assured the PLO representative in Paris that France is prepared to meet this request.

Both the Lebanese Moslems and Arafat fear that without an international presence, the Israelis and their Lebanese Christian militia allies could stab them in the back, seize West Beirut and subject both Moslems and Palestinians to their diktat.

As soon as the international force, which would also include American, Greek, Canadian and Italian contingents, began arriving, the timetable said, Palestinians pledged they would regroup. They no longer demanded that the regrouping take place in their refugee camp strongholds.

Over the following two days, Salam said, those guerrillas leaving for Jordan and Iraq would depart either by the Beirut-to-Damascus highway to Syria or, for those on the outs with the Damascus regime, from two Lebanese airports, Rayack in the eastern Bekaa Valley or Qlaiaat north of Tripoli.

The day after that, and for three additional days, those Palestinians headed for Egypt and other, unspecified countries would leave, either by ship from Tripoli or by road via Damascus.

Once that operation was completed, from the seventh until the 14th and final day of the exercise, the bulk of the forces would travel by road to Damascus, which technically has always been the PLO's official headquarters. For guerrillas in transit through Syria, the timetable said, their stopover there would be completed in 24 hours.

Both Salam and Palestinian sources reported that Habib insisted that Arafat and the other Palestinian leaders remain in Beirut until the very end of the evacuation.

News agencies added:

Most of the staff of the French Embassy in West Beirut was evacuated to East Beirut after the embassy was hit by Israeli artillery, slightly injuring three people, Foreign Ministry officials said in Paris.

Israeli shells also hit the West Beirut headquarters of the Armenian Secret Army for the Liberation of Armenia on July 30 and killed its guerrilla leader, known by his nom de guerre Hagop Hagopian, a statement by the group said.

The leaders of moderate North Yemen and radical South Yemen made it known that their trips to Saudi Arabia and Syria earlier this week resulted in a tentative agreement to hold an Arab summit on Lebanon next week.