In a move that sent new tremors of fear through the two Palestinian refugee camps where hundreds of civilians were massacred by Christian militiamen less than a month ago, the Lebanese Army today sealed off and searched the camps house by house.

The sweep of the tense camps was the latest step in a city-wide operation that has been going on for days aimed at uncovering hidden arms depots and arresting criminals and illegal aliens.

Lebanese security sources have reported the detention of more than 500 persons, but other, unofficial, sources put the total at more than a 1,000. Some have been released after an identity check and others deported. But hundreds are still being held, among whom are reported to be many Palestinians.

The arrest of Palestinians, in some cases by armed civilians identifying themselves as Lebanese security men, has stirred fears throughout the Palestinian community of a second wave of persecution by the authorities. The government has already made clear that it wants the vast majority of the estimated 500,000 Palestinians in Lebanon to leave as soon as possible.

The anxiety gripping the Palestinian camps of Sabra and Shatila was clearly evident during a visit there today. Armored cars and soldiers of the Lebanese Army were everywhere, and at mid-morning an officer standing in a Jeep was shouting through a bullhorn for everyone to stay at home.

A group of angry Palestinian women asked to speak to several French soldiers and U.N. observers standing nearby. They said their husbands and sons had been taken away earlier in the day, even though they were 1948 refugees with papers allowing them to live in Lebanon.

Both a French officer and a U.N. observer said there was nothing they could do because it was "an internal matter" involving the Lebanese Army and security in the city.

One woman was nearly hysterical and shouted at the soldiers and whoever else would listen, "I have no lights, no water, no electricity, and now they come and take my son and husband. Who is going to feed me?"

Before she could say more, a soldier pushed the group away with the side of his gun and told them all to go home. Photographers and television teams were banned from the camps today, apparently because of Army fears of such scenes.

It was not known tonight how many had been arrested in today's sweep of the camps, which have already been the scene of intensive hunts for wanted persons by the Lebanese Army and other security authorities.

Various persons interviewed, including a French officer, told of seeing between 60 and 100 Palestinian men taken away early in the morning in one part of Sabra camp.

The French officer, head of one of the units stationed in the camp as part of the international peace-keeping force, denied reports that armed civilians had entered the French-patrolled area of Sabra camp during the night to arrest a number of Palestinians.

There have been local press reports during the past few days of various incidents in Sabra, Shatila and Burj al Barajinah camps, where Palestinians have been picked up by armed civilians suspected of being Christian militiamen.

Lebanese Moslem sources have estimated the number of such mysterious arrests at around 150 persons in the past few days and accused Christian militiamen of carrying out the operations.

Tonight, the Internal Security Force admitted that police in civilian clothes had entered Sabra and Shatila yesterday but said they had done so to arrest seven Palestinians involved in a robbery east of Beirut. Two of the Palestinians were arrested and confessed to the crime in complicity with the other five still at large, the security force said.

The communique announcing the operation said orders had been given to security men not to enter the camps again except in their uniforms.

Some press reports have said the Christian Lebanese militia are acting in cooperation with the regular army and have given it a list of Palestinians and Moslem leftists they want apprehended. But there has been no confirmation of these reports.

It was clear in one instance today, however, that one of the Army officers checking identity papers in Sabra camp had a list of wanted persons.

Meanwhile, reports published in the local press today said the Army will not begin its house-by-house search of Christian East Beirut until Oct. 22 or 23 after President Amin Gemayel returns from his visit to the United States, where he is scheduled to meet with President Reagan.

This is widely regarded as the first real test of Gemayel's authority because the operation will involve the disarming of the Christian militiamen based in the eastern sector.

Officials of the Lebanese Forces insist they will obey whatever orders Gemayel issues regarding the fate of the militiamen in East Beirut and elsewhere. But considerable doubt remains among West Beirut's Moslem population that the Lebanese Forces will disband without opposition.