The Lebanese Army has been rounding up hundreds of Palestinians, spreading new terror in the camps where the massacres took place last month, in what appears to be a government effort to reduce the number of Palestinians here by about 90 percent.

The arrests have raised fears among the residents of the Sabra and Shatila camps, this time about their future status in Lebanon and whether they are destined to undergo daily harassment from Lebanese authorities seeking to drive them out of the country.

The government has leaked to the local press a plan, still under discussion, that would reduce during the next few years the number of Palestinians allowed to live here from about 500,000 to 50,000.

Even more indicative of the official Lebanese attitude is the fact that the government has not approved repeated requests from the U.N. agency responsible for the Palestinians to set up tents, even on a temporary basis, in any of the destroyed camps.

The exact number of Palestinians arrested is difficult to determine because some have been released after a check of identity papers, while others have been expelled from the country or taken to the Israeli detention center in southern Lebanon at Ansar. Furthermore, it is not just Palestinians but all foreigners who are being picked up, including nationals from Pakistan, Bangladesh, Turkey and various Arabs who have taken advantage of the chaos in Lebanon to live here for years without residence permits or with forged papers.

The respected newspaper An Nahar quoted an Army spokesman as saying 578 "illegal aliens" had been arrested in a raid on the Burj al Barajinah camp. But many others who do not fit into that category have been detained.

One Western diplomatic source said he thought that about 1,000 Palestinians and other foreigners had been detained.

Army spokesman Maj. Atef Torbay said the roundup was part of the Army's reestablishment of its authority in the capital, and he conceded, "People are going to be somewhat affected at first."

"It's normal in this kind of situation," he said, that "some incidents" occur. But he insisted that the Army had entered the Palestinian camps to "protect the people" and said those being held would be treated "according to the Lebanese law."

"The Army is doing its best to protect them," he said repeatedly, adding, "We are not a totalitarian country, and we are going to establish a free and democratic society."

The wave of arrests, which began before the return of the international peace-keeping force but is still continuing, has placed the French and Italians stationed in the camps in a difficult position because part of the force's mandate is to provide security in the camps. On the other hand, as one Italian diplomatic source noted, they are also here to help restore the authority of the Lebanese Army and to interfere would undermine this goal.

Wednesday, former prime minister Saeb Salam met with President Amin Gemayel to express his concern on behalf of West Beirut's Moslem leaders about reports of 1,500 "missing persons."

Reports circulating in the capital estimate the number of arrests among Palestinians living in Sabra, Shatila and Burj al Barajinah, the three camps where the peace-keeping force is stationed, at somewhere between 400 and 600 during the past two weeks.

A Western diplomatic source said some detainees were seen being taken in trucks to Sofar, on the highway to Damascus, and dumped there. Sofar is where the Syrian line of control in eastern Lebanon begins.

The roundup of Palestinians is also taking place in camps around Sidon, the main city in southern Lebanon still under Israeli occupation. There, it is the Israelis rather than the Lebanese Army making most of the arrests.

Thursday, at least 100 persons were taken from Ain Hilwe camp, on the eastern outskirts of Sidon. After a preliminary interrogation at the Israeli headquarters, about half were put into trucks and driven to Ansar, according to a foreign relief worker who looked into the matter but asked that his name and agency not be mentioned.

The arrests there were triggered by the shooting of what was described by local residents as an "Israeli agent." They said the unknown gunmen fled immediately.

On Tuesday, panic swept the Ain Hilwe camp after gunmen, thought to be rightist Christian militamen, killed a 60-year-old Palestinian walking in the street.

The incident set off a panic among the refugees, many of whose homes still fly a black flag in memory of the Sabra and Shatila massacre victims.

Those deaths plus news of the wave of arrests in the camps here in West Beirut have set the Palestinian community on edge and fearful of its immediate security and future.

The deep feeling of anxiety gripping Palestinian refugees in the camps was made clear during a brief visit to Shatila yesterday.

When it became clear what the two Americans were interested in, they were led through a warren of streets to a back alley where men and women poured out of their homes to talk.

Within half an hour, the small group provided the names of at least 10 arrested Palestinians and even more names were offered.

Almost all of the older residents interviewed said they were refugees who arrived in Lebanon in 1948 with sons born here since then. The men arrested were of all ages and not all had been taken from the camps. Several, they said, had been arrested at Lebanese Army checkpoints in the city center and all have valid residence permits.

Ali Ahed Hilou, 58, said he was picked up Sept. 21 by Lebanese Army soldiers as he sat in the street near his home. He was taken to an Army barracks on Badaro Street in East Beirut with 30 to 40 others, he said.

Hilou said hundreds of Palestinians and a few Lebanese were brought in while he was there and they slept in cramped quarters without blankets on the floors in several rooms. Among those brought in later were his two sons, aged 16 and 27.

Some Palestinians told reporters of being hit, but Hilou said he had been relatively well-treated and only made to answer questions about what he was doing in Lebanon and whether he had ties to the Palestine Liberation Organization or any other political groups.

He was eventually taken to the Lebanese Defense Ministry at Yarze, again with others. He said he was asked the same questions and finally released after being held 10 days. But his two sons, one on a visit from Saudi Arabia, where he works, were still there, he said.

Compared to the refugees in Ain Hilwe and nearby Mieh-Mieh outside Sidon, those in Shatila may consider themselves lucky, since they at least have the peace-keeping force present and no one is shooting at them.

Thursday, Ain Hilwe residents were on edge during a visit there because Israeli troops had just ordered all males older than 12 to report to the hospital at 5:30 a.m. About 100 were taken away for questioning.

The apparent reason was to find who was responsible for the shooting of the alleged "Israeli agent." But the Israelis used the roundup to cull 50 Palestinians and send them to their detention center in Ansar.

The center now houses 8,000 persons, and the number has been increasingly steadily since mid-August, despite the release of more than 600 detainees, according to informed sources. About 100 of those now being held there were taken at the time of the massacre in Sabra and Shatila camps Sept. 16-18, they said.