The raid on the southern village of Qana was the bloodiest single attack during Israel's 19-day-old war on Hizbollah.

As a wave of anger spread across Lebanon and the Arab world, several thousand protesters chanted "Death to Israel, Death to America" outside the United Nations headquarters in downtown Beirut and some smashed their way into the building.

U.S. Secretary of State Condoleezza Rice canceled a visit to Beirut on Sunday, saying she had work to do in Israel to get a truce.

Rice, who returned to the region on Saturday in a new push to bring the warring sides together, said she was "deeply saddened" by the Israeli bombing of the village of Qana in southern Lebanon, killing 40 civilians including 23 children.

"In the wake of the tragedy that the people and the government of Lebanon are dealing with today, I have decided to postpone my discussions in Beirut. In any case, my work is here today," Rice told reporters, adding that she had canceled the trip and not the other way around.

She spoke after Lebanon's Prime Minister Fouad Siniora said, after the latest Israeli bombing, he could not hold any talks on resolving the Middle East crisis before an immediate ceasefire.

"There is no place on this sad morning for any discussion other than an immediate and unconditional ceasefire as well as an international investigation into the Israeli massacres in Lebanon now," Siniora told a news conference in Beirut.

Police, who gave the death toll, said the Israelis had bombed Qana at 1:30 a.m. (2230 GMT on Saturday), destroying a three-storey building where about 63 displaced people were sheltering in the basement. Many were killed in their sleep.

"Why have they attacked one- and two-year-old children and defenseless women? What have they done wrong?" asked Mohamed Samai, whose relatives were among the dead.

Hizbollah vowed to retaliate. "This horrific massacre will not go without a response," it said. The governing Palestinian movement Hamas also pledged to hit back with attacks on Israel.

Rice said it was "time to get to a ceasefire", but she insisted this required changing the status quo before the war, which began after Hizbollah guerrillas seized two Israeli soldiers in a cross-border raid on July 12.

At least 542 people have been killed in Lebanon in the war, although the health minister estimated the toll at 750 including unrecovered bodies. Fifty-one Israelis have also been killed.

Many Arab and European leaders condemned the Qana bombing and called for an immediate ceasefire. Siniora called U.N. chief Kofi Annan to demand an emergency Security Council meeting.

But Israeli Prime Minister Ehud Olmert told his cabinet the assault in Lebanon would go on. "We will not blink in front of Hizbollah and we will not stop the offensive despite the difficult circumstances," the Ynet site quoted him as saying.

Olmert told the cabinet of his "deep sorrow" at the civilian deaths in Qana. Political sources said he had also ordered that humanitarian aid be allowed to reach the village.

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