Lebanon's seven main political and military leaders met for the first time tonight as the new National Salvation Council.

A functioning council is considered essential to efforts by Lebanon's hostile factions to bring peace to their country. Its meeting brought hope to war-weary Lebanese that the threatened Israeli assault on their capital can be averted.

Holding of the long-delayed meeting was a major achievement for U.S. special envoy Philip C. Habib, who worked with Lebanese authorities throughout the past week to get the council into operation.

The first order of business of the closed session almost certainly was the deployment of the 21,000-man Lebanese Army inside the capital. The troops' mission was to maintain order and to oversee the disarming of Palestinian guerrillas and their Lebanese allies.

The Army's deployment was a major barrier to getting the council functioning after its creation last Monday. The fact that the body had finally met seemed to indicate that the seven members already had reached a measure of agreement.

Sources familiar with the discussion said, however, that there was a sharp split over how to deal with the Palestine Liberation Organization. Some members reportedly demanded the PLO's unconditional surrender, while others sought to preserve a political position for the PLO and to obtain international guarantees that it would not be attacked by either Israel or the Lebanese Army if it turned in its arms.

Following the 3 1/2-hour meeting, Prime Minister Shafiq Wazzan said the first session was held in "a good, brotherly and frank atmosphere with similar viewpoints expressed regarding proposed solutions."

He refused to give any further details but said the council would meet again within 48 hours.

Wazzan said the fact that the meeting had taken place reflects optimism and "augurs well" for decision-making. "It reflects the determination of the Lebanese to save Lebanon, starting with relieving it from the Israeli occupation and ending its 8-year-old crisis," he said.

President Elias Sarkis, a Maronite Catholic, heads the council. Members are three Moslem leaders--Wazzan, a Sunni; Nabih Berri, a Shiite, and Walid Jumblatt, a Druze--and three other Christians--Foreign Minister Faud Butros, a Greek Orthodox; Bashir Gemayel, a Maronite, and Nasri Maalouf, a Greek Catholic.

It brings together the leaders of Lebanon's three largest private militia forces--Gemayel's Christian Lebanese forces, Berri's Shiite Amal Movement and Jumblatt's National Movement.

The body has no formal constitutional powers and does not replace the existing government. But the general assumption here is that if its members can agree on action to rescue the country, the Cabinet will ratify its decisions.

Sources said tonight that Gemayel, supported by Sarkis, Wazzan and Butros, were demanding virtually the unconditional surrender of the Palestinians.

Jumblatt, on the other hand, has gone on record as saying, "I will not be the one to give the coup de grace to the Palestinians."

There were strong indications, however, that PLO leaders were resigning themselves to a new status for their forces in Lebanon despite a spate of defiant declarations during the past 48 hours.

Saeb Salam, 77, a former prime minister and a key mediator between Habib and PLO leader Yasser Arafat, told a group of reporters at his home this morning that he was convinced the Palestinians had "no desire to continue as a militant military power like before."

But, he cautioned, "you cannot ask the Palestinians to throw down their arms at our feet," and said "they want to maintain their diplomatic status here."

One PLO source said Arafat was seeking two conditions for laying down arms. First, he wanted the Israelis to withdraw at least 10 miles from the capital, and second, he sought guarantees from Saudi Arabia and the Soviet Union that his guerrillas would not be attacked by either the Lebanese Army or Israel.

Altogether, the indications were that some deal may be in the works that would transform the PLO into a mainly political movement with little or no armed force.

This, Lebanese leaders hoped, would be sufficient to avert a bloody Israeli invasion of West Beirut to crush the last 5,000 to 6,000 guerrillas trapped here and destroy the PLO's leadership.

In addition, Salam said he had "the impression" from his many talks with Habib during the past week that the United States is willing to put pressure on the Israelis to withdraw from the capital area once the Salvation Council is functioning and the Lebanese Army had begun deploying in the capital.

Nevertheless, Salam and outside analysts seemed aware that the whole plan may go awry.

Salam was particularly suspicious of Israeli Defense Minister Ariel Sharon, prime architect of the invasion and particuarly the Israeli drive to reach the Lebanese capital.

"I cannot imagine Sharon refraining from any extreme excess if he has the chance for it," Salam said.

The convening of the Salvation Council is a landmark in the bloody history of Lebanese politics since the 1975-76 civil war. Since then, the main political and militia chiefs of Lebanon's various communities have not been willing to sit at the same table.

Gemayel, 34, the tough head of the Maronite militia, has never before been accepted by the Moslem leaders as one of the chief official spokesmen of the Christian community in national politics. Observers here say his presence on the council will reinforce his aspirations as a national leader and future candidate for president.

Aside from the difficulties of getting an agreement among the seven members on the Salvation Council, it remains to be seen whether more radical factions of the PLO than Arafat's Fatah organization will go along with any plan calling upon the guerrillas to throw down their arms.

Salam said that if Arafat makes the decision, "I think it will be applied." But George Habash, leader of the Popular Front for the Liberation of Palestine, said a few days ago that "it is impossible for us to lay down our arms."

Meanwhile, the PLO, apparently anxious to show its readiness and determination to oppose an Israeli invasion of predominantly Moslem West Beirut, took several hundred correspondents and television teams here on a tour of their defenses near the airport.

The journalists went through a maze of dirt embankments, mined roads and gun emplacements in what are now the largely deserted Palestinian camps on the southern outskirts of the city.

The sound of two outgoing artillery rounds could be heard nearby and a helicopter, presumably Israeli, whirred in the distance. Otherwise all was quiet.