U.S. and Lebanese negotiators, seeking to break a deadlock over an American plan to send trapped Palestinian guerrillas out of Beirut, met for a second day of discussions today with no apparent sign of progress amid reports that Israel has moved up two brigades to the outskirts of the capital.

A cease-fire that ended fierce artillery and rocket exchanges Sunday night held through the day. But two days of relative tranquillity in Beirut were shattered this evening when a car bomb went off outside the Palestinian Research Center in the predominantly Moslem half of the capital, killing at least one person and injuring 30.

An informed military source said there was speculation among military experts that Israel may seek to put pressure on the Palestine Liberation Organization by driving up the Bekaa Valley in eastern Lebanon against Syrian and Palestinian forces there in a classic operation at which the Israelis excel. That would put the Israelis in a position to besiege the northern port of Tripoli, the PLO's last major stronghold outside Beirut.

An all-out assault on West Beirut increasingly is being viewed here as too costly, if not impracticable, for the Israelis. It was not immediately clear whether the arrival here of an armored brigade and a mechanized brigade sent up the coastal road from Israel amounted to a reinforcement or simply a rotation of the estimated 25,000 to 30,000 Israeli troops besieging the PLO, the military source said.

U.S. special Middle East envoy Philip C. Habib conferred again with President Elias Sarkis, Prime Minister Shafik Wazzan and Foreign Minister Fuad Butros at the Baabda presidential palace.

One obstacle to a political settlement continues to be Syria's decision Friday not to accept Palestinian guerrillas evacuated from Beirut. Negotiators here are looking to the planned visit to Washington of the Saudi Arabian and Syrian foreign ministers to solve the problem, but there has been confusion about the date of their trip.

In Washington, informed sources said said the visit was scheduled for this weekend and a White House official said that a "mutually convenient time" was being sought for a meeting of the two officials with President Reagan.

A Lebanese government official close to the negotiatons also said a "deadlock" persisted over the question of whether an international peace-keeping force should enter Beirut before the Palestinians withdraw, as the PLO demands, or afterward, as the United States and Israel insist.

The PLO says the force must be in place beforehand to ensure the disengagement of forces, the safe withdrawal of the guerrillas and the protection of Palestinians who stay behind in refugee camps. The United States apparently fears that if the force comes in before a pullout, the Palestinians will feel assured of a buffer against the Israelis and will continue to delay their departure.

The Associated Press quoted a political adviser to PLO Chairman Yasser Arafat as saying the PLO would be more flexible in the talks if the United States would grant it the recognition it has long sought. "I am sure that if there were direct talks between the PLO and America through Philip Habib, it would be a very important step for the PLO, and the PLO would be ready to be more flexible and to rethink a lot of things," Hani Hassan was quoted as saying.

Boutros was quoted as saying that one of the key topics at the meeting with Habib was the Israeli blockade of West Beirut. The 11-day-old blockade, which has been leaky throughout, eased today as fresh fruits and vegetables were plentiful for the first time since it was imposed.

The two-day-old cease-fire was broken briefly this afternoon by a small amount of shelling from PLO positions north of the airport.

Israel is believed to have at least 300 tanks and 500 to 600 armored personnel carriers on the outskirts of the city, in which about 5,000 to 6,000 Palestinian guerrillas are bottled up.

According to the military source, the Israelis currently have at least eight divisions in Lebanon totaling about 120,000 troops.

Tripoli is believed to offer an attractive alternate target to the Israelis because only a couple of thousand guerrillas are protecting about 30,000 Palestinian refugees in camps.

It would apparently be easier to capture than Sidon and Tyre, which were taken only after devastating bombardment.

Although Israel has repeatedly demanded the departure of the PLO from Lebanon, it has virtually ignored the presence of Palestinian guerrillas in Tripoli, according to political observers here.

Besides the danger of taking unacceptably high casualties in an assault on Beirut, the Israelis may lack enough professional, highly trained troops to do the job, military sources said. They pointed out that the vast majority of the Israeli ground troops are draftees or reservists.

The threat of an Israeli assault on West Beirut having faded somewhat, the PLO seems to be more confident that it can resist the Israeli siege.

This confidence was boosted Sunday when, despite intense Israeli shelling, the guerrillas knocked out several Israeli tanks near the airport and destroyed six trucks in a rocket attack on an Israeli logistics base southeast of the city, military sources said. They said the Israelis evacuated the base yesterday.