Following high-level Arab-U.S. talks in Washington, Palestine Liberation Organization officials here express the conviction that Lebanese and American negotiators are coming around to at least some of the PLO's terms for evacuating guerrillas from Beirut.

Basing their optimism mainly on preliminary accounts of the Syrian and Saudi foreign ministers' talks concluded yesterday, the PLO officials also make clear that the concessions they expect from the Americans are not major ones.

According to a well-informed PLO source, the Palestinian optimism is tempered by fears that Israel could launch a major strike to upset any developing agreement between Washington and the PLO. Fueling these fears were expressions of Israeli concern that the United States stand on not recognizing the PLO was eroding and reports of an Israeli military buildup south and east of Beirut.

Amid the signs of growing Israeli impatience and repeated violations of the 10-day-old cease-fire between Palestinian and Israeli forces, indications are now emerging that the PLO might be prepared to moderate one of its main conditions for starting its evacuation: receiving some form of political compensation from the United States, such as direct contacts or recognition, that would advance the Palestinian quest for a homeland.

According to senior Lebanese and Palestinian officials, the United States is insisting that the PLO must begin its evacuation before it can expect any such political payoff. And even so, U.S. officials are said to have made clear that no guarantees would be given. The PLO is said to recognize that Israel cannot be expected to acquiesce to PLO conditions for withdrawal if it must swallow U.S. recognition of the PLO at the same time.

The PLO source hinted that enough progress had been made in winning the Americans over to the PLO's thinking on the evacuation timetable that the guerrillas no longer felt obliged to stress their demands for political recognition.

The demands for political recognition appear to be dictated by a need to prepare Palestinian public opinion for abandoning Beirut as a major base of operations, according to analysts. The United States, they reasoned, could hardly be expected to reverse overnight its longstanding hostility toward the PLO, especially while the Israelis held the upper hand militarily.

As contacts among Palestinian, Lebanese and American negotiators continue here, public pressure has been mounting for Israel to end its 17-day-old blockade of West Beirut, where 5,000 to 6,000 Palestinian guerrillas are trapped inside a ring of Israeli troops and armor. Several hundred protesters led by Moslem clerics stormed past a PLO checkpoint today, approaching within a couple of hundred yards of Israeli tanks to demand an end to the siege.

Earlier, Palestinian forces in West Beirut's southern suburbs near the airport traded machine-gun and heavy-weapons fire with Israeli troops dug in with tanks and artillery on the hills to the east.United Press International quoted the Israeli military command as saying guerrillas fired a rocket into Galilee, causing no damage or injury.

Israeli artillery barrages resumed late tonight, with the sound of explosions heard coming from the Palestinian strongholds near the airport.

According to independent military sources, the Israelis have brought up dozens of rocket launchers and artillery pieces just south of the airport. Numerous trucks loaded with ammunition were seen on the coastal road, with some of the shells out of their cases and ready for firing.

Despite the potentially ominous signals on the military front, Palestinian sources expressed optimism on the diplomatic developments, reporting that the Saudi and Syrian foreign ministers had found President Reagan favorably disposed toward the PLO evacuation plan in their meeting yesterday. In addition, the Kuwait news agency reported from Washington that Reagan was amenable to Palestinian proposals for withdrawing from Beirut to other points in Lebanon on their way out of the country, the introduction of a multinational peace-keeping force and an initial, partial, Israeli withdrawal.

"I think the Americans have begun to come around," a well- informed PLO official here said. He acknowledged, however, that he had no hard facts on which to base this impression and that Washington may be softening its line on the PLO evacuation terms to sidestep the harder issue of political recognition.

The PLO has been insisting on an 11-point plan for evacuating its forces and lifting the Israeli siege of the Lebanese capital. According to PLO officials, the points include a disengagement of forces, a five-mile pullback by Israeli troops around Beirut, withdrawal of Palestinian forces into their camps, the introduction of an international force and Lebanese Army units to protect the camps and supervise the disengagement, negotiations between the Lebanese government and the PLO on the details of the Palestinian presence in Lebanon, and the evacuation of any Palestinian forces beyond the number that the PLO and Lebanese agree may remain.

However, faced with Lebanese government resistance to the idea of putting off the crucial negotiations on the Palestinian presence in Lebanon, the PLO has agreed to begin setting out details of its withdrawal, Palestinian sources said.

The United States and key elements of the Lebanese government have been insisting that the Palestinians evacuate Beirut before the introduction of an international peace-keeping force and the departure of the Israelis. Now, according to a PLO source, the Lebanese government is coming around to the idea that the international force come in first, that the Palestinians and Israelis disengage in stages and that "all final withdrawals be simultaneous."

Former prime minister Saeb Salam, a key mediator in the current negotiations here, said the Reagan administration showed "some understanding" during the visit of the Saudi and Syrian foreign ministers of the need to end the siege of Beirut, obtain "the withdrawal of the invaders" and resolve the Palestinian question. He said that although the meetings yielded mostly "atmospherics," there was "evidence of openness."

A top PLO official, Hani Hassan, said, "we are happy that President Reagan did not try to squeeze us."

Hassan's older brother, PLO executive committee member Khaled Hassan, is currently in Washington, where he has been dealing indirectly with the Reagan administration for the past few days, PLO officials here said. Officials in Washington insist there is no contact with Hassan.