An emergency meeting of the UN's new human rights body on the Gaza crisis has heard condemnation of Israeli policy and its effects on civilians.
Israel has ordered its army to widen its operations
Israel was violating in Gaza the "most fundamental norms of humanitarian law and human rights law", UN envoy John Dugard told the special session.
But Israel, which threatens to broaden military action, said the Human Rights Council was ignoring its concerns.
The meeting was called by Muslim states with support from Russia and others.
UN aid agencies warn of a humanitarian crisis in the territory as Israel confronts the Hamas-led government.
Food and medicines are running low, and there have been power and water shortages since the Israelis began military action last Tuesday.
Bridges have been bombed along with the Strip's only power station.
Aid workers had cautiously welcomed the debate in Geneva but feared it might become bogged down in a political slanging match, the BBC's Imogen Foulkes reports.
'Bombarded and terrorised'
Mr Dugard, the keynote UN speaker, told the session that Israel's conduct was "morally indefensible".
The crisis between Hamas and Israel has gripped Gaza for days
"Over 1,500 rounds of artillery have been showered on Gaza," he said.
"Sonic booms terrorise the people. Transport has been seriously disrupted by the destruction of roads and bridges. Sanitation is threatened."
While expressing sympathy for the Israeli soldier whose capture sparked Israel's offensive last week, he said that Israel's military action was a "disproportionate use of force against civilians".
Mr Dugard, a UN special rapporteur on human rights in Gaza, also accused the so-called Quartet of Mid-East mediators - the US, EU, UN and Russia - of failing to rein in Israel.
Itzhak Levanon, Israel's ambassador to the UN in Geneva, said the session was a "planned and premeditated" attack on his country.
And he suggested the newly created Council was continuing the alleged anti-Israeli bias of its discredited predecessor, the Human Rights Commission.
"We find ourselves in an absurd situation in which the Human Rights Council, convened into urgent session, ignores the rights of one side and holds a special meeting to defend the rights of the other side," he said.
The United States has also suggested that the Council is returning to what they see as the bad old ways of the discredited Human Rights Commission.
That body often focused on Israel's alleged human rights abuses, while ignoring serious violations in other parts of the world, our correspondent notes.
The special session may run into Thursday, diplomats say.
Given the situation in Gaza, there was never much chance that the subject would be ignored by the UN's top human rights body, the BBC's Imogen Foulkes in Geneva says.
The request submitted by the Organisation of the Islamic Conference - a group of 57 largely Muslim states - argued there was a "serious and escalating situation" in Gaza.
The resolution being debated calls on "Israel, the occupying power, [to] end its military operations in the occupied Palestinian territory... and refrain from imposing collective punishment on Palestinian civilians".
Meanwhile, Israel's security cabinet instructed the army to begin a long battle focusing on "institutions and infrastructure facilitating terrorism" in response to a Palestinian rocket attack on the Israeli city of Ashkelon.