The Israeli Army thinned out its forces in southern Lebanon today, withdrawing most of its mechanized infantry units into the narrow, Israeli-declared "security zone" after six days of unsuccessful searching for two Israeli soldiers captured by Moslem militiamen, the Army command said.

Officials of the U.N. peace-keeping force in southern Lebanon said there was heavy traffic of Israeli military equipment toward the international frontier, but that it was impossible to tell how many of the estimated 3,000 Israeli troops that participated in the search operation had actually left Lebanon.

An Army command spokesman in Tel Aviv confirmed that Israeli forces had interrogated 2,000 suspected Lebanese guerrillas, and were still holding "between 50 and 100" of them in connection with Monday's ambush and abduction of the two Israeli soldiers near Kounine.

The soldiers were seen being driven away in a black Mercedes-Benz sedan, and their photographs have since appeared in Beirut publications, with claims by the Islamic Resistance Front, which includes the radical Shiite Moslem militia Hezbollah (Party of God), that one of the soldiers has been executed in reprisal for the Israeli incursion into southern Lebanon.

Israeli military sources said that 80 Hezbollah militiamen were still being held by Israeli troops and militiamen of the Israeli-controlled South Lebanon Army.

During the week-long incursion, the search for the missing soldiers gradually evolved into a massive hunt for weapons and an operation that appeared designed to disrupt the infrastructure of the various Shiite Moslem militias operating in southern Lebanon, north of the Israeli-declared security zone.

The Israelis encountered stiff resistance on several occasions, and two Israeli servicemen were killed during the operation. In one clash last night, according to Israel's state radio, one Moslem guerrilla was killed and four SLA militiamen were wounded.

Seven men from the town of Shaqra, just north of the Israeli border, charged today that they and scores of others had been beaten by SLA forces during interrogations behind a school in the town, special correspondent Nora Boustany reported from Beirut. The seven, taken by U.N.forces to a hospital in Tibnine, showed bruises and burns in films made by ABC-TV and the European Vis-News Co. The film contained the first direct charge of maltreatment, Boustany reported.