Palestinian leader Yasser Arafat tonight signed a detailed document agreeing to the evacuation of the Palestine Liberation Organization's troops and political leaders from Lebanon, according to a key negotiating intermediary.

Former Lebanese Prime Minister Saeb Salam told The Washington Post that he had telephoned U.S. special envoy Philip C. Habib with the news and said that the contents of the document are expected to be sent immediately to Israel. Salam refused to provide any details.

Despite Salam's enthusiasm, other sources close to Lebanese Prime Minister Shafiq Wazzan and Arafat were less optimistic about the reported accord and feared that the Israelis would find that it did not go far enough.

In Washington, Reagan administration officials were unavailable for comment. But an informed diplomatic source in Washington said that the document, signed after a five-hour meeting of the leadership of the Palestine Liberation Organization in Beirut, provided answers in writing to questions that had been presented to the Palestinians by Habib, through the Lebanese, on June 23.

U.S. Ambassador to Israel Samuel Lewis met with Prime Minister Menachem Begin on the eve of an Israeli Cabinet meeting scheduled for Sunday morning, correspondent Edward Cody reported from Tel Aviv. No details of the meeting were announced in Israel, but official sources in Washington said that it was held to review the status of negotiations in Beirut and to ask for more time to arrange a negotiated end to the armed Palestinian presence there.

The apparent breakthrough in the deadlocked negotiations came after Israel today tightened its hold on West Beirut by stopping incoming traffic to the besieged predominantly Moslem western sector of the capital and exchanged artillery fire with Palestinian forces around the city's shantytown southern suburbs.

Salam said that the "written, detailed" document, signed at his home at about 9 p.m. [3 p.m. EDT], "implies the specifics" of the timing and conditions of the Palestinian evacuation for which Wazzan had been pressing Arafat during the past week.

Informed sources, while providing few details, said the document calls for an international or multinational military force to assist the Lebanese Army in policing the capital and the departure of the PLO organization leaders and troops.

Asked if such a force would involve U.S. troops, the diplomatic source in Washington said that the United States is "keeping its options open."

Salam said this afternoon that the four groups involved in the negotiations--Americans, Israelis, Palestinians and Lebanese--had made progress by agreeing "in principle while not definitively" that an international force should help the Lebanese Army in Beirut after the Palestinians left. He suggested that the 7,000-man U.N. Interim Force in Lebanon, stationed in the south, could be brought in for the task. Specialists said the force could be in Beirut within 24 hours.

Arafat was expected to hand the document to Wazzan tonight, and the prime minister was expected to take it early Sunday morning to Habib at the residence of the U.S. ambassador in the Israeli-occupied hills overlooking the capital.

One worrisome aspect, according to government sources, was that the document did not nail down the Palestinian request for a temporary, token military force--said to have been whittled down from an original demand for a permanent force in the thousands to two 250-man battalions that would leave Lebanon upon the departure of the last Syrian and Israeli troops.

Instead, they said, the document left that question to be negotiated in the next 48 hours with the Lebanese authorities.

To some observers, Arafat seemed to be once again playing for time, trying to avert the threatened Israeli ground assault.

In his comments earlier today, Salam, a six-time prime minister, had complained wearily that the Israelis, by continually boosting their threats and indicating an attack was imminent, were not allowing enough time for the complicated negotiations. Echoing the fears of many in West Beirut, he said, "If the Palestinians are squeezed, Israeli Defense Minister Ariel Sharon will have the opportunity to destroy this place and you and me with it.

"I cannot gauge the plans of that criminal planner, Sharon," he added.

All sides had agreed that the principal obstacle to a settlement remained the Palestinians' refusal to write down on paper with the Lebanese government exactly what they were willing to do to bring about their safe conduct out of Lebanon, and where they would go.

The Israelis have promised the approximately 5,000 guerrillas the right to take their personal arms with them, and Salam has said the Israelis also are willing to pull back from their present lines once the Palestinian evacuation has commenced.

But, as Salam suggested earlier today, the Palestinians prefer to stay in Lebanon, where they enjoy more freedom of movement and expression than they could hope for elsewhere in the Arab world.

A Lebanese politician, describing this dilemma, said, "If they go to Syria, it's like going to jail; to Algeria, its oblivion; Egypt, it's disarming and a government in exile; to Jordan, King Hussein's united kingdom plan; to South Yemen, it means swimming."

Speaking to reporters today, before the signing of the agreement, Arafat said, "Habib asked me to leave. To where? The only place to go is Palestine."

The establishment by Israel of roadblocks between the eastern and western sectors of the city today--coming amid the first major cease-fire violation in Beirut in eight days--was seen as the possible start of an effective blockade of food, fuel and other essentials into the western sector. The blockade and the fighting were seen as part of the increasing psychological warfare tactics designed by the Israelis to increase pressure on the negotiators here.

Salam charged that the Israelis had "broken the cease-fire twice" during the day and said he "twice was in contact with Habib, and Habib promised to stop it."

Israeli Army spokesman Barry Swerski told reporters in the hills above the capital that the measures were "indicative of what we can do."

Amid widespread speculation throughout most of the day that Israel would unleash some military operation after the Sunday morning Cabinet meeting, Arafat told reporters this afternoon that he expected an attack from three divisions of Israeli troops plus gunboats and war planes "within 24 hours."

Today's artillery, mortar and machine-gun duels between Israelis in the hills overlooking the city and Palestinians on the plains near the airport lasted two hours. Israeli bulldozers dug out hillside positions for tanks just above the southern suburbs.

And in the mountains east of Beirut, eyewitnesses reported that Israeli artillery in Sofar on the strategic Beirut-to-Damascus road traded barrages with Syrian gunners in and around Hammana.

Military specialists said that the Israelis, with more than 400 tanks and 100 artillery pieces ringing West Beirut, have the ability to easily launch a limited attack or a major, final onslaught against the Palestinian positions in West Beirut.