Panic swept through the Moslem section of Beirut today as rumors spread that Christian militias responsible for last week's massacre were returning to the refugee camps, and thousands of Palestinians fled from the camps before the Lebanese Army could restore calm.

What triggered the panic in West Beirut is unclear but some said it was the appearance of uniformed men -- later identified as Lebanese Army regulars -- near the Shatila and Sabra refugee camps, where hundreds of residents died Thursday and Friday when the militias moved into the camp. Others said it was the sound of explosions coming from mine-clearing operations by the Lebanese Army.

Nonetheless, the renewed fear spread through jittery Beirut like wildfire. Thousands of men and women, many clutching babies and dragging crying children, rushed to the northern section of the city to seek refuge in hospitals, the International Red Cross headquarters, city parks and even the home of former prime minister Saeb Salam.

"Haddad is back, Haddad is back," was a constant cry from the distraught refugees flooding the commercial sector of Beirut. They were referring to former Lebanese Army major Saad Haddad, who controls Israeli-supported forces in southern Lebanon. Although Israel and Haddad have denied he was involved in the massacre, witnesses have described the attackers as members of Haddad's forces and the Christian Phalangist forces based in Beirut. The Phalangists have also denied any involvement.

It took hours for Lebanese Army troops with loudspeakers and Israeli troops using bullhorns to convince the Palestinians not to fear. The Army, once suspected by the Palestinians of being a tool of the Maronite Christian factions, was greeted today with applause by the refugees as its troops took up positions near the camps.

The Israelis, who denied complicity in the attack but who let the militias cross their lines to move into the camps, continued to thin out their ranks in West Beirut today. Lebanese officials said they fear that the situation in Beirut could become more violent now if the Israelis withdraw after disarming most of the Moslem leftist militias that remain in the city while the Christian militias continue to take more control.

The government today requested a new multinational peace-keeping force in the city, and President Reagan said later that the American, French and Italian forces that had supervised the withdrawal of the Palestine Liberation Organization fighters last month would return to help restore control.

Troops from both the Phalangist "Lebanese Forces" militias and Haddad's militia have been seen inside West Beirut in recent days, often near Israeli Army positions, and have begun setting up their own roadblocks in the city center during the tense nightly curfews that Israel imposed on the city.

Israel moved into West Beirut after the assassination last Tuesday of president-elect Bashir Gemayel, the leader of the Phalangist forces and an Israeli ally. Gemayel's brother, Amin, has been nominated by the Phalangists as a candidate to replace Bashir in elections scheduled for Tuesday.

Amin Gemayel's leading rival, 82-year-old former president Camille Chamoun, withdrew from the race today as it became obvious that Gemayel had the votes needed to succeed his brother. The only other candidate is Raymond Edde, who has been living in France since 1976 and is not considered a threat to Gemayel. Outgoing president Elias Sarkis' six-year term expires Sept. 23.

Observers had speculated that Bashir Gemayel, who had a reputation as a tough, commanding leader, would be able to unite Lebanon's long divided Christian and Moslem communities. But it appeared that to accomplish that unity Gemayel would have to disband all local Christian and Moslem militias to allow a rebuilt Lebanese Army to impose government authority.

Worried Lebanese and Christian politicians said they believe the massacre shows that after Bashir's death the Lebanese Forces are splintered and not under the control of Amin, leading to fears of more instability. They point to the fact that Amin acknowledged to U.S. diplomats that his forces were among those going in shortly after the movement of the militias became known here. Amin assured the diplomats that his troops would be leaving soon, but there is no indication that they did.

The Phalangist units originally from the Christian town of Damur were seen leaving the Shatila refugee camp after the killings were over. The Damur Christians were driven out of their town, which is about 18 miles south of Beirut, in 1976 as Palestinians and Lebanese Moslems retaliated for Christian assaults against a Moslem slum in Christian East Beirut and a Palestinian refugee camp. Six months later, as many as 3,000 Palestinians died in Lebanese Forces' attacks at Tal Zaatar refugee camp.

The Damur Christians formed their own battalion of the Lebanese Forces after they fled to East Beirut, and because of their hatred for the Moslems who had driven them from their homes, were always considered one of the most fanatic units of the Gemayel militia.

They were trained extensively by the Israelis, and Western diplomats here suspect they may have formed a special link with the Israeli Army, much like that of Haddad's militia, which was created by Israel to be a southern buffer at the Israeli-Lebanese border.

"As long as Bashir Gemayel was alive he probably could have held his militiamen's loyalty and discipline," said one European diplomat. "But Amin Gemayel commands none of the same personal loyalty and respect, and it is clear now that he has lost control of the units that took part in the Shatila massacre."

At the camps today, Red Cross workers and volunteers continued to pick through the rubble looking for bodies. They said 130 bodies have been recovered and none of the mass grave sites have been opened.

In Jeddah, Saudi Arabia, visiting PLO chief Yasser Arafat said in a television interview that more than 3,200 people had been killed in the two camps, including almost 1,500 in two hospitals, Reuter reported.

He repeated earlier charges that the multinational force was partly to blame for the slayings because it left earlier than expected.

The force was established in the cease-fire agreement negotiated by U.S. special envoy Philip C. Habib to end the Israeli siege of Beirut and the evacuation of PLO fighters.