The special UN envoy for the Middle East, Alvaro de Soto, said he was "deeply shocked and appalled" by the shelling of civilian homes by Israeli forces.
In Cairo, the Arab League denounced the Palestinian deaths as a massacre and called an emergency meeting of foreign ministers from its 22 member nations.
Amr Moussa, the organisation's secretary general, said: "These massacres of children, women and civilians are unjustified and incomprehensible and unexpected.
"Israeli policies in the Palestinian territories have gone too far."
Sheikh Hamad bin Khalifa al-Thani, the Emir of Qatar, called Ismail Haniya, the Palestinian prime minister, to condemn the attack.
Despite being a regional ally of Israel, Turkey was also critical.
The foreign ministry said in a statement: "Israel's disproportionate and indiscriminate use of force against [Palestinian] rocket attacks is not conducive to permanent security and stability in the region.
"On the contary, it ... fans the spiral of violence and mutual enmity and leads to the indefinite postponement of possible peaceful coexistence," the statement said.
Moussa has called an emergency
meeting of the Arab league
Margaret Beckett, the British foreign secretary, said she was deeply disturbed by the deaths of Palestinians in the Gaza Strip, saying it was hard to see how the Israeli strike could be justified.
"I am gravely disturbed by the deaths of Palestinian civilians, including women and children, in an Israeli strike on Beit Hanoun in the Gaza Strip," she said in a statement.
"I call on all sides to meet their obligations under international humanitarian law and to do their utmost to avoid harming civilians, especially children."
In Brussels, Benita Ferrero-Waldner, the EU's external relations commissioner, said the shelling by Israel of civilian homes in Beit Hanun was "a profoundly shocking event".
"Israel has a right to defend itself but not at the price of the lives of the innocent," she said.
"This brutal collective punishment of a people, not a government, has passed largely unnoticed by the international community"
John Dugard, UN human rights rapporteur
The failure of the international community to stem the bloodshed in the Middle East has come in for sharp criticism from the UN's special rapporteur on human rights in the occupied Palestinian Territories.
"This brutal collective punishment of a people, not a government, has passed largely unnoticed by the international community," John Dugard said in a statement.
"The Quartet ... has done little to halt Israel's attacks."
"Worse still, the [UN] Security Council has failed to adopt any resolution on the subject or attempt to restore peace to the region. The time has come for urgent action on the part of the Security Council," he said.
There has also been a widespread call for Palestinian groups to show restraint, with Moscow pointing out that firing rockets at Israeli territory would only provoke further military action.