The estimated 700 to 800 Lebanese Shiites held in an Israeli prison camp, whose release is demanded by gunmen holding TWA Flight 847, have become a central factor in the hijack drama despite an oft-expressed Israeli intention to free them soon in any case.

There was no apparent change today in the position voiced late last night by well-placed Israeli sources, that Israel would not negotiate for the release but would consider a U.S. request to free them to spare the lives of hostage Americans.

That account reinforced the widespread impression here that Israel would agree quickly to the release of the Shiite prisoners if it could take that step in response to a formal and public U.S. request, not yet forthcoming, that would place the responsibility for agreeing to the hijackers' demands on Washington.

Israel's holding of the Shiites had become an international issue before the hijacking. Last April 2, the Israeli Army transferred more than 1,000 prisoners from its Ansar Prison Camp in southern Lebanon to prisons inside Israel.

The move was made necessary because the Israelis, gradually withdrawing from Lebanon, were about to evacuate the Ansar area and dismantle the camp.

At its peak, in the summer of 1982, Ansar held more than 10,000 prisoners, almost all of them Palestinians swept up in the first weeks of the Israeli invasion of Lebanon. By the end of the Lebanon war, the camp's population was not only much smaller but different in its ethnic makeup.

Most of the prisoners who were moved in April to the Atlit Prison on the Israeli coast south of Haifa were Lebanese Shiite Moslems. These are the men whose release is being demanded by the hijackers of the TWA aircraft in Beirut.

For the Israeli government, the TWA hijacking incident has come amid particularly uncomfortable circumstances that are feeding pressures to harden its stand.

Last month, Israel agreed to release 1,150 Arab prisoners, most of them Palestinians and several of them convicted terrorists, in exchange for three Israeli soldiers captured during the war in Lebanon.

The decision brought charges here that Israel was bending in its traditionally tough anti-terrorist stand and, since last Friday, numerous assertions that the hijackers of the TWA aircraft were probably encouraged to believe they would succeed by Israel's agreement to free so many prisoners for so few Israeli soldiers.

If anything, the TWA hijacking appears to have hardened public opinion in Israel against the release of Arab prisoners under any circumstances.

Israel ended up with so many Lebanese prisoners because the nature of its conflict in Lebanon changed during the three-year occupation. Most of the Palestinians, who were the targets of the 1982 invasion and captured during the first weeks of the war, had been freed by this year in a series of major prisoner exchanges with various factions of the Palestine Liberation Organization.

But by late 1984 and early 1985, the local population in southern Lebanon, led by the Shiite militia Amal, was engaged in a deadly resistance to the Israeli occupation. The attacks grew more fierce as the Israelis began their prolonged, staged withdrawal from Lebanon in February.

Israel reacted as it always has in such situations: it retaliated. In a series of military sweeps, dubbed Operation Iron Fist, through the Shiite villages in the mountains of southern Lebanon, the Israelis rounded up hundreds of men and teen-agers, searched houses, and destroyed the weapons and explosives they found.

There was a standard operating procedure: the Israelis arrived at dawn, surrounded the village, ordered the men to go to the village school and the women to remain in their homes. The men were questioned and some of them detained.

How the Israelis decided on which men to detain has never been explained. But almost all the prisoners were natives of the villages of southern Lebanon. From the beginning, Operation Iron Fist was defended by the Israelis as a necessary defensive measure to reduce the number of attacks on their withdrawing soldiers. The Israelis never expressed an interest in holding the Shiite prisoners indefinitely, and in the wake of the TWA hijacking and the demand for their immediate release, Israel has stated its policy explicitly:

The remaining prisoners will be released gradually, as some Lebanese Shiites already have been freed, depending on the "security situation" in southern Lebanon, according to Israeli officials. They maintain this had been made clear to Amal before the hijacking. One reason the Israelis have little interest in holding onto the Shiite prisoners is that they are of unlikely use even as "bargaining chips" in some possible future prisoner exchange. Amal and other purely Lebanese organizations have never held Israeli prisoners to trade.

When Israel moved the prisoners from Ansar to Atlit in April, it added a new reason to rid itself of the Lebanese Shiites. The transfer was in violation of international law, as Israeli officials concede. The Shiites are not prisoners of war but civilian detainees, and the Fourth Geneva Convention prohibits the transfer of civilians to the territory of an occupying power.

Both at Ansar and during their confinement in Israel, the Shiite prisoners have been accessible to representatives of the International Committee of the Red Cross. It is also known from an interview with one of the prisoners that they are able to send and receive some mail from their families in Lebanon.

Both the United States and Israel have prided themselves on not agreeing to the demands of terrorists, a position the two governments have congratulated each other on over the years. But in this case, the two governments find themselves in conflict, for while it is American lives that are at stake in Beirut, the demand for concessions is being put directly to Israel by the hijackers.