Palestine Liberation Organization leaders today reached a formal agreement "in principle" with the Lebanese government to leave Lebanon with most of their guerrillas and sent the accord through U.S. diplomatic channels to Israel for a response.

The breakthrough, which came during prolonged discussions between PLO and Lebanese officials, brought hope once again--despite some unresolved issues--that the threatened Israeli assault on West Beirut to crush the remaining guerrillas bottled up here could be avoided.

But Lebanese leaders said much still depended on the United States and whether the Reagan administration was willing to use its full influence to moderate Israeli terms for what is seen here as a humiliating unconditional surrender of the 5,000 trapped Palestinian guerrillas.

To back up its demands, Israel, for the second straight day, sent warplanes over West Beirut to drop leaflets urging residents to leave to save their lives. But the cease-fire that went into effect Friday night remained largely intact.

The PLO, which up until early this morning was denying reports of an agreement and pledging a fight until the death, said little publicly about today's developments, which were outlined by the state radios of both Lebanon and Israel and confirmed in most details by State Department sources.

But speaking privately or in less specific terms today, PLO officials acknowledged movement in the negotiations and did not deny local press reports that appeared with banner headlines on the agreement in principle. They tried to fudge the issue, however, saying "all kinds of proposals" were still under discussion.

"Nothing is yet final but we are approaching the final," said one PLO official who requested anonymity.

Salah Khalaf, the number two PLO official, speaking on a left-wing radio station, said, according to Reuter: "We have presented reasonable proposals that the Israeli Army should withdraw 5 or 10 kilometers after which a meeting would take place between us and the Lebanese authorities to regularize the Palestinian presence." He gave no details.

The Palestinians were reported to be delighted by the way the Israeli government had addressed its conditions directly to the PLO Sunday, interpreting this as a "quasi recognition" of the organization.

This was said by one diplomat in contact with PLO officials today to have played a role in the late night offer by the Palestinians to withdraw from Lebanon.

The latest Israeli demands are that the guerrillas turn over their arms to the Lebanese Army and, with their leaders, leave Beirut for Syria, passing through Israeli lines with the promise of a safe passage and under the escort of the International Red Cross.

PLO leader Yasser Arafat, however, was demanding that the guerrillas be allowed to leave Lebanon with at least light arms and with guarantees from the United States, the Soviet Union and Lebanese Christian leader Bashir Gemayel that they would not be shot at, according to Lebanese sources who attended last night's meeting.

Lebanese leaders, showing signs of optimism for the first time in a week, agreed that time had become a main factor now in the complicated four-party negotiations involving the Palestinians, Lebanese, Israelis and Americans.

"We are racing with time," said Saeb Salam, the former Lebanese prime minister who has been serving as a conduit between the Palestinians and U.S. special envoy Philip C. Habib. "Time is precious and more time is needed to clarify and do things."

A similar assessment came from Marwan Hamadeh, the Lebanese tourism minister and a Gemayel associate who negotiated with the PLO leaders last night. Today he said, "The real manager of time now is the United States."

Salam said he had appealed to Habib to get the Reagan administration to become "a bit more flexible and reasonable" and "to lean on Israel" to moderate its demands toward the PLO and relieve its pressure on the siege of the western sector of the city.

"Let's hope there will be some echo" from Washington, he said to reporters at his home today.

Israeli Prime Minister Menachem Begin, he said, was out to get "the last pound of flesh" from the Palestinians and he warned that this attitude could provoke a reaction from the PLO leadership of preferring to fight to the death here.

He said Habib had sent two messages back to Washington Sunday regarding the Lebanese plea for time and moderation but expressed doubt he would obtain a favorable response even after the resignation of Secretary of State Alexander M. Haig Jr., a strong supporter of Israel.

Salam said he feared there were still too many "small Haigs" throughout the administration to bring about any substantive change in U.S. policy toward Israel.

But he said both Saudi Arabia and Egypt, the United States' two closest Arab allies, were now letting Washington know in strong terms what it would cost the Reagan administration in its relations with them if the Israelis launched an attack on West Beirut.

Israel acted to keep up its pressure on the city today, again dropping leaflets urging residents to "hurry up and save your life" and flee while they could.

"Time is beginning to run out," the leaflets said. "The longer you delay, the greater the danger becomes to your safety and the safety of your beloved ones. Thousands of your brothers have seized the opportunity given them and left West Beirut and are now living in freedom and security."

Sunday, Israeli jets dropped their first warnings over West Beirut, causing a small stampede into the Christian eastern sector. But the panic subsided today with news of a nearing accord on the Palestinians' departure.

Aside from the Israeli jets and Palestinian antiaircraft fire to chase them away, the cease-fire arranged by Habib Friday night continued to hold throughout today with no major violations reported.

But Israeli forces were said by Lebanese Army sources to be taking advantage of the pause in the fighting to inch their way along a mountain road parallel to the vital Beirut-to-Damascus highway toward Syrian positions in the Metn valley region.

Meanwhile, two sets of negotiations appeared to be under way simultaneously here today in the search for a solution to the problem of a new status for the PLO in Lebanon and a withdrawal of its leaders and guerrillas from the country.

On the one hand, PLO and Lebanese officials continued to hold talks throughout the day on what Salam called "the practical means" of accomplishing the Palestinian departure from West Beirut in a manner acceptable to the guerrillas and their leaders.

The two sides were also reported to be hammering out the new status of the PLO in Lebanon, with the Lebanese understood to be willing to guarantee it the same diplomatic status it now enjoys with Arafat accorded treatment as a chief of state.

The other issue was the status and treatment of the 600,000 Palestinian refugees living in Lebanon.

Lebanese and diplomatic sources said the PLO leadership offered last night to quit the capital and country with 5,000 guerrillas for a destination yet to be determined.

It was not clear tonight where the figure 5,000 had come from. A high-ranking PLO official said in an interview with The Washington Post today that the PLO had regular Palestinian soldiers from the various brigades it has serving under the Syrian, Jordanian and Egyptian armies that it wanted to get back to their host countries.

Among the main issues still unresolved was reportedly a Palestinian proposal to the Lebanese for the establishment of a "symbolic and regular" military presence under Lebanese authority.

Local press reports said the PLO was still asking for a residue battalion, similar to the Yarmouk Brigade under Syrian Army command, that would serve under the Lebanese Army.

But Lebanon was reported to have rejected this, saying it was having enough trouble working out a balance between Christian and Moslem Lebanese in the Army without adding a disruptive outside force.

Another key issue is what arms the departing Palestinian guerrillas will be allowed to take with them. PLO leaders were said to be demanding that the guerrillas leave with at least light arms.

The Lebanese position on this was not clear but it was thought the government had no objection.

The third major question, which the PLO is obviously using as a means of negotiating with Israel, was the method and route of the guerrillas' departure from Beirut.

"The Palestinians want to find a peaceful corridor literally and figuratively," said one high-ranking Lebanese official involved in the talks.

There were conflicting reports today as to whether the PLO preferred to leave by land or sea. Salam said that the idea of one big convoy of buses or a small armada of boats was not "practical" and was regarded by the guerrilla leaders as dangerous to their security.

But one PLO official told a diplomat that ships were on their way here to take the guerrillas to Syria, Egypt and perhaps Libya.

If they left by land, the guerrillas would have to pass through the Christian sector of Beirut if they wanted to avoid passing through Israeli lines. This would explain the report that Arafat was asking Gemayel's Lebanese Christian forces for a guarantee of safe passage.

The other set of negotiations today was between the Israelis and Palestinians through the intermediary efforts of Lebanese Prime Minister Shafiq Wazzan and Habib.

The proposals agreed upon between the Palestinians and Lebanese last night were forwarded to Habib through Wazzan today.

The main stumbling block now appears to be the Palestinian demand for an Israeli pullback around the capital and for the establishment of a "symbolic and regular" force to remain behind after the bulk of the guerrillas depart.

The Israelis have repeatedly said they want all Palestinian guerrillas disarmed and the PLO turned entirely into a political movement, making it seem unlikely they will accept even a "symbolic" Palestinian armed wing under Lebanese Army command.

The Israelis have also refused to withdraw from the capital area before the Palestinians disarm and leave West Beirut.

One high-ranking PLO official said today that Habib had talked about a possible "readjustment of forces" if and when there was an agreement between the PLO and the Lebanese government. That has apparently now been mostly achieved, but it remains to be seen whether the United States will now work for such a "readjustment" that would satisfy the Palestinians.