QANA, Lebanon - Israeli shells killed at least 75 Lebanese refugees yesterday, filling a U.N. camp with blood, horror and survivors' cries for revenge. Israel admitted an ''unfortunate mistake'' in the attack, which seemed certain to deepen further hatreds in the Middle East.

The carnage, which left bodies torn apart, was the worst since Israel began its onslaught against Hezbollah guerrillas in south Lebanon eight days ago. While expressing regret for the civilians' deaths, Israel fiercely defended its Lebanon campaign and said the shells that hit the civilians had been aimed at Hezbollah rocket launchers.

President Clinton called for a cease-fire by all parties to the fighting and ordered Secretary of State Warren Christopher to the region to mediate.

Israel said it would accept a cease-fire if others agreed to it, a move that would leave Israel short of its goal of shutting down the Hezbollah war machine.

The Israeli attack left the U.N. base littered with butchered and headless bodies, shredded clothing and scraps of building materials. Badly wounded people were rushed to a hospital, where angry civilians attacked three Hezbollah members, beating them with sticks and chairs and accusing them of being the source of Lebanon's misery.

Timur Goksel, spokesman for the 4,500-strong U.N. peacekeeping force in Lebanon, confirmed that 75 people perished in the Qana attack.

Lebanese leaders called the Israeli shelling ''the mother of all atrocities'' and a new page in ''the annals of terror.''

Israeli Prime Minister Shimon Peres said, ''I'm pained by every person, every woman, every child, who is being killed.''

But he said Israel had ''no choice but to defend its citizens,'' and he accused the Iranian-backed guerrillas of Hezbollah of hiding behind civilians.

The United Nations said that shortly before the Israeli shells landed, Hezbollah guerrillas about 300 yards from the U.N. compound had fired two Katyusha rockets and eight mortar rounds at the Israelis. The Israeli shells apparently were in retaliation for that fire, but missed their target.

Unlike the hundreds of thousands of Lebanese who fled north when Israel warned it would attack their villages, the 6,000 refugees at U.N. bases had elected to stay in the south, believing they were safe among the peacekeepers. About 500 refugees were at the U.N. base in Qana.