The recently created Human Rights Council voted on Thursday to condemn the offensive launched last week, which Israel said was to recover a soldier captured by Palestinian militants on 25 June.
The 47-member council adopted a resolution demanding a halt to Israel's military operation and decided to "dispatch an urgent fact-finding mission by the Special Rapporteur on the situation of human rights in the Occupied Palestinian Territory".
The special session on Wednesday and Thursday was called by the Arab Group in the UN and supported by about a dozen other member states, including China, India, Russia and South Africa.
Five countries abstained from the vote, in what was the council's first special session, as attempts to broker a consensus failed.
Despite expressing concern about the impact of the offensive, Western countries opposed the resolution or abstained, complaining of bias because it did not consider the kidnapping of an Israeli soldier that preceded military action, or to urge a halt to attacks by Palestinian militants on Israelis.
The US, which is only an observer at the council, called on all parties in the Middle East to exercise restraint.
"What you have in front of you is a very mild and diluted resolution"
Mohammad Abu-Koash, Palestinian ambassador, UN
Warren Tichenor, the US ambassador, said: "A historic opportunity to address the human rights situation in a fair, equitable and balanced way has instead resulted in an unbalanced effort to single out and focus on Israel alone."
The resolution expressed "grave concern at the violations of the human rights of the Palestinian people caused by the Israeli occupation, including the current extensive Israeli military operations".
The council called for "Israel, the occupying power, to immediately release the arrested Palestinian ministers", other MPs and "all other arrested Palestinian civilians".
The text also urged "a negotiated solution to the current crisis".
In text added at the last moment in an unsuccessful attempt to placate European concerns, the resolution urged "all concerned parties to respect the rules of international humanitarian law and to refrain from violence against civilians".
Mohammad Abu-Koash, the Palestinian ambassador, told the council: "While we gather here in this hall, Israeli tanks are moving and shelling Palestinians, the office of our parliament has been besieged by Israeli troops.
"What you have in front of you is a very mild and diluted resolution."
Both sides in the conflict were also urged to "treat under all circumstances all detained combatants and civilians in accordance with the Geneva Conventions".
The special rapporteur, John Dugard, regularly reports on the situation in Palestinian territory and was there last month. Israel has refused to co-operate with him or his predecessors.
Dugard told the council that the offensive in Gaza was "morally reprehensible" and amplified a human rights situation that was already "appalling".
The South African lawyer said that he hoped that the captured Israeli corporal, Gilad Shalit, "is well treated as a prisoner-of-war and released in good health".