Israeli forces, suddenly facing pockets of stiff resistance in their drive to pacify Moslem West Beirut, cordoned off that sector of the city today and began a systematic hunt for Lebanese and Palestinian sympathizers of the now-evacuated Palestine Liberation Organization.

Plainclothes security agents, carrying lists of names, led squads of soldiers through the streets in search of presumed enemies to interrogate. Scores of suspects were forced to wait for hours before being questioned, although only small numbers were seen being led away afterward.

Armed members of the Lebanese Forces, the Christian rightist militia that is informally allied with the Israelis, entered Palestinian refugee camps on the city's southern outskirts and went from house to house arresting Palestinian males. In several instances, Christian and Moslem militiamen exchanged rockets and gunfire.

Cordons of Israeli tanks surrounded the Shatila, Sabra and Burj al Barajinah camps as the Christian militiamen -- feared and hated by the Moslem population since the brutal 1975-1976 civil war -- carried out their work.

Lebanese Moslem leaders again protested that the United States had broken a pledge to block Israel from occupying West Beirut.

[In Jerusalem, Israel said it was willing to open discussions with the Lebanese Army "as soon as possible" on a troop pullback from the city, Washington Post correspondent Edward Walsh reported. The statement came after U.S. special envoy Morris Draper met with top Israeli officials there. Details on Page A10]

Israeli soldiers apparently ignored diplomatic traditions in their sweep through the heart of West Beirut. Until U.S. diplomats ordered them out, they installed themselves in the half-completed building that is to be the new U.S. Embassy on Ramlet el Baidah Avenue. They also fired a shot at a Marine guarding the U.S. Embassy but missed him. An Israeli official later apologized, saying the guard had been mistaken for a leftist militiaman.

Other Israeli soldiers entered and occupied for two days parts of the Soviet Embassy compound despite the protests of its diplomats, according to Soviet officials and Israeli soldiers talking to news agency reporters. The Israelis set up command posts for their Army in the embassy of the United Arab Emirates as well as in U.N buildings occupied a week ago in the southern neighborhood of Bir Hasan.

Reuter quoted Soviet sources as saying that about 60 Israeli soldiers in two armored personnel carriers smashed through a metal gate in the side wall of the embassy compound on Wednesday night. Israeli soldiers said that they moved into the compound to command a view of the strategic Corniche Mazraa boulevard, Reuter said.

In Jerusalem, a military spokesman confirmed that Israeli soldiers had entered the Soviet compound Wednesday but denied that they had forced their way into a building, Reuter said. "There was shooting in the area, and some of our men took shelter behind a wall in the embassy compound for a limited time," the spokesman said.

ABC news reported that Soviet Ambassador to the United States Anatoliy Dobrynin said, "I don't have details, but it is clearly an act of aggression, which we clearly will not tolerate."

President Reagan, asked to comment on Dobrynin's statement as he left an Italian-American festival in Flemington, N.J., said, "You know those Russians; you can't believe anything they say." Deputy White House press secretary Larry Speakes said the White House still was getting "conflicting reports" about exactly what happened at the embassy.

The Israeli military command announced that one Israeli soldier had been killed and 46 wounded during the past 24 hours.

In a surprise political development, the National Liberal Party nominated its 82-year-old leader Camille Chamoun to run for president against the older brother of Bashir Gemayel, the president-elect who was assassinated Tuesday. Both Chamoun and Gemayel's brother Amin, who was nominated yesterday by the Phalangist Party, are Christian leaders.

The official Lebanese government radio reported that Raymond Edde, another Christian, has become the third candidate for the Lebanese presidency as head of the National Bloc party, Agence France-Presse reported.

Although fighting was continuing, there was little doubt that the heavily armed Israeli forces eventually would quell resistance being put up by small, disorganized bands of young Lebanese militiamen fighting with Kalashnikov assault rifles, antitank rocket-propelled grenades and a few recoilless cannons mounted on jeeps.

But it was not the futile fighting that most worried West Beirut's half million residents. It was, instead, the prospect of a sectarian crackdown on the Moslem population by Israel and its Lebanese Christian allies.

Fears were stirred particularly by the sudden deployment of the Israeli-armed Christian militiamen, known locally as the "kataeb." The militiamen -- who were led previously by Bashir Gemayel -- are remembered with bitterness for their massacre of up to 3,000 Palestinians in the now-razed Tel Zaatar refugee camp in 1976 at the height of the Lebanese civil war.

Although a tight Israeli Army security cordon tried to keep outside observers from the Palestinian refugee camps in the southern suburbs today, there were reports by civilians who managed to escape of violent reprisals by the militiamen. One Palestinian woman who fled to the house of former prime minister Saeb Salam reported in tears that her husband's throat was slit while she watched.

Salam, echoing comments by Lebanese political leaders yesterday, said that "Israel has made a laughing stock of U.S. honor and credibility" by invading West Beirut.

The Lebanese government insists that the accords negotiated by Philip C. Habib not only provided for the PLO evacuation but also for Israel's withdrawal from its positions around the city as soon as the PLO completed its withdrawal Sept. 1. Officials rejected Israel's claims that it had to enter West Beirut to maintain order following Gemayel's killing and that the PLO had broken the accords by leaving behind 2,000 guerrillas.

"We counted on American guarantees, we took it at its word and thought it had the power and the will to meet its responsibilities," said Salam, who was a key intermediary between Habib and PLO chief Yasser Arafat in the negotiations. "Instead we see it is not the United States that leads, but Israel."

In the Habib agreement, Israel accepted the number of 7,100 PLO combatants to be evacuated from West Beirut along with about 2,500 of their Syrian Army and Syrian-controlled Palestine Liberation Army allies. In the evacuation itself, about 8,300 PLO combatants left the country, according to U.S. counts, and 3,600 Syrian troops and PLA fighters also departed.

One Lebanese Cabinet minister said that Israeli Defense Minister Ariel Sharon, in talking now about another 2,000 PLO guerrillas, was using a "disingenuous excuse to justify an invasion that he had already planned."

This morning in Beirut, Israeli Army jeeps mounted with loudspeakers moved down the city's commercial artery, Hamra Street, ordering merchants to shut down their shops and everyone to go to their homes.

Civilian security agents, heading squads of soldiers, went from neighborhood to neighborhood in the city ordering all men to come into the streets for interrogation and, eventually, to turn over any weapons they might have in their houses.

A group of about 175 was thus herded on the seafront corniche near the U.S. Embassy. The civilian in charge interrogated the group about names he had on lists of men suspected by the Israelis. Only two men, a Palestinian and a Lebanese belonging to a local militia group, actually were seen being hustled away in an Israeli Army truck.

One Western diplomat witnessed a similar scene in the middle-class apartment house district of Raouche. There, about 130 men were forced to sit on a sidewalk for several hours while being interrogated. In the end, six or seven men were detained and taken off to an unknown destination.

Yousef Nazal, the millionaire proprietor of the Hotel Commodore, where most of the Western press corps stays, was taken at gunpoint from his hotel lobby for brief questioning by Israeli officials who asked him why the PLO had held press conferences in his hotel. He told them that the press conferences were held there because the press lived there.

A 6 p.m.-to-6 a.m. curfew was clamped on the city.

In the camps to the south, a foreign television camera crew that managed to get to Sabra camp's Gaza hospital filmed children and old men and women who had been wounded badly by shrapnel. The crew interviewed a doctor who said that more than 1,000 people had been killed and wounded in the camps during the past two days. There was no way to confirm the report.

Spokesmen for the Lebanese Forces militia in East Beirut denied any knowledge that its men had invaded the camps that had once been bastions of the PLO. But this correspondent, after evading Israeli Army roadblocks and making two separate visits to Shatila during the day, saw the militiamen operating openly there.