The emergency meeting, requested by the 118 UN member states making up the nonaligned movement, got under way after an Israeli bid to block it on procedural grounds was dismissed.
"Gaza is ablaze. It has been turned into a burning hell," said the assembly president, Nicaragua's Miguel d'Escoto Brockmann. "The violations of international law inherent in the Gaza assault have been well documented: collective punishment; disproportionate military force; attacks on civilian targets, including homes, mosques, universities, schools."
UN Deputy Secretary General Asha-Rose Migiro told the 192-member assembly that UN chief Ban Ki-moon, currently on a visit to Israel, conveyed "his strong protest and outrage" and demanded an explanation after Israeli shells smashed into a UN compound in Gaza, setting fire to warehouses holding badly needed aid.
Other Israeli strikes set a hospital wing on fire and wounded two cameramen in a building housing international and Arab media outlets.
Prime Minister Ehud Olmert said that his troops had shelled the compound in Gaza in response to fire coming from the building.
Speaker after speaker blasted the latest Israeli attacks.
D'Escoto deplored the fact that the Israeli offensive, launched to stop rocket firing by Palestinian militants and now in its 20th day, was continuing despite a cease-fire call issued by the powerful 15-member Security Council last week.
'Israel in contempt of international law'
"It seems to me ironic that Israel, a state that more than any other owes its very existence to a (1948) General Assembly resolution, should be so disdainful of United Nations resolutions," D'Escoto noted.
"Prime Minister Olmert's recent statement disavowing the authority of Resolution 1860 clearly places Israel as a state in contempt of international law and the United Nations," he added.
D'Escoto urged the assembly to agree on a non-binding resolution reflecting " the urgency of our commitment to end this slaughter" in Gaza.
Malaysia's UN Ambassador Hamidon Ali said the proposed resolution should call for a cease-fire, the immediate withdrawal of Israeli forces from Gaza, an end to the Israeli blockade of the Palestinian enclave and allowing humanitarian aid to reach victims of the conflict.
"It must also call for the establishment of a tribunal to investigate and to prosecute those responsible for the war crimes and crimes against humanity," the Malaysian envoy said. "Finally, sanctions must be imposed for non-compliance."
The assembly session could run though Friday.
Israel has conditioned ending its war on Hamas in Gaza on international guarantees that the Islamists wouldn't be able to rearm by smuggling arms from Egypt and on a halt to rocket fire from the enclave.
Since Israel unleashed its Operation Cast Lead on December 27, at least 1,073 people, including 355 children, have been killed and at least another 5,000 wounded, according to Gaza medics.
Palestinian UN observer Ryad Mansour meanwhile called for an independent investigation of Israel's "grave breaches and systematic violations of international law."
"Since this crisis began, it is without a doubt that a multitude of war crimes have been perpetrated by the occupying power (Israel)," he said while also calling for "measures for protection of the defenseless Palestinian civilian population."