For the second time in two years, Israeli and Lebanese officials sat down across from each other at a triangular negotiating table today in a renewed attempt to end the Israeli occupation of southern Lebanon. Military delegations from the two countries and from the United Nations met in a one-story, prefabricated building that normally serves as a conference hall at the headquarters of the U.N. Interim Force in Lebanon (UNIFIL).

Following the four-hour session, including a lunch of chicken, roast beef, salad, rice, Israeli yogurt and Lebanese wine, the delegations issued a bland joint statement that said they would meet again on Monday.

In the negotiations with the Lebanese, Israel is seeking security arrangements in southern Lebanon as a precondition for a withdrawal of its troops from the territory they have occupied since the June 1982 invasion.

The Israeli objectives include an expansion of the area patrolled by U.N. troops, the continued deployment of the Israeli-backed South Lebanon Army in southern Lebanon and the right of Israeli forces to reenter Lebanon to assist the South Lebanon Army.

Few details emerged from the first day of negotiations, in which the two sides reportedly restated their positions in some of the procedural squabbles that preceded the conference. Lebanon has sought to portray the talks as being held under U.N. sponsorship, while Israel maintains that they are "direct," with the U.N. officials acting as observers.

Timur Goksel, the UNIFIL spokesman here, said, "Both sides made their points. If there were disagreements, they were recorded, but the conference went on."

Goksel said that the two sides agreed to meet three times a week beginning next week, and that he expected the session on Monday to begin dealing with the substance of the issues.

Two fundamentalist Moslem organizations in Lebanon have condemned the talks and threatened retaliation against the participants. Security was tight as the negotiations opened, with a new barbed-wire fence ringing the conference hall while Israeli soldiers in full combat gear and U.N. troops were stationed at the entrances.

The talks, which could go on for months, are part of a two-track negotiating process the Israelis have embarked upon to achieve security guarantees for their northern border. They were arranged through the mediation of U.N. Middle East specialist Jean-Claude Aime and are expected to center on security arrangements following an Israeli withdrawal.

While these negotiations are taking place, Israel hopes the United States will act as a mediator in indirect contacts with Syria. Israel is seeking assurances from Syria that it will not move into positions vacated by the Israeli Army after a pullout from Lebanon and that it will prevent Palestinian guerrillas from moving south toward Israel from territory controlled by Syria.

The Reagan administration has not committed itself publicly to act as a mediator with Syria, although Assistant Secretary of State Richard W. Murphy, who met yesterday with top Israeli officials, continues to travel in the Middle East to explore that possibility.

The Syrians are not represented here, but they still hold the key to the success of the negotiations and to Israel's hope of withdrawing from Lebanon after gaining security guarantees.

Syria pressured Lebanon earlier this year into abrogating the first agreement with Israel for a troop pullout, the May 17, 1983, accord that was signed after almost five months of negotiations that began in December 1982.

Israeli officials say Syria's willingness to have the talks open here today is an indication of Syrian interest in achieving an agreement. At the same time, the Israelis acknowledge that the process could be lengthy and that there is no guarantee that in the end Syria will be willing to meet Israel's minimum demands.

The 12-member Lebanese delegation flew from Beirut aboard two U.N. helicopters. The delegation was headed by Brig. Gen. Mohammed Haj, the military commander of the Beirut area.

His counterpart on the Israeli side was Brig. Gen. Amos Gilboa, who is assigned to the Army's planning branch and is a former senior intelligence officer. The 10-member delegation included the only woman to take part, Lt. Col. Margalit Meital.

The United Nations was represented by Lt. Gen. William Callaghan, the commander of UNIFIL; Callaghan's senior civilian adviser, Ramon Prieto, and three other military officers.

An Israeli Army spokesman described the atmosphere as "direct, open and friendly." Another Israeli official said it was not realistic to expect the negotiations to produce quick agreement. Publicly, senior Israeli officials have suggested that they expect the talks to last at least three months. They also have said there will be no unilateral Israeli pullback until all efforts to achieve security arrangements through negotiations with the Lebanese have been exhausted.

The key stumbling block may be the South Lebanon Army. Both Syria and Lebanon, which consider the force to be an Israeli puppet, have strongly objected to any continuing role for it in Lebanon after an Israeli withdrawal.

Brig. Gen. Antoine Lahad, the commander of the South Lebanon Army, told Israeli radio today that he saw little hope of success in the negotiations. The radio quoted Lahad as saying that the Lebanese government had no real solutions to the security problems of southern Lebanon and that UNIFIL was "ill-equipped" to handle the job.