"In defending itself, Israel is also also defending many of the countries whose delegates just left this hall," Netanyahu said.
"In fact, I want you to know that many of their leaders even thank us privately for taking out Iran's nukes."
When Netanyahu walked to the rostrum, a mix of cheers, boos and mass walkouts erupted.
Scores of delegates trooped out and a majority of seats sat empty afterward.
"Order in the hall, please," General Assembly president Khalilur Rahman said repeatedly.
"If there are any other moral cowards who have not yet left the hall, please do so now," Netanyahu said.
The Palestinian Mission to the UN said in a message to many of the 193 UN member countries to bring as many staff as possible to the assembly hall and "collectively walk out of the assembly" once the Israeli prime minister is announced.
"This will send a clear message to Netanyahu and his government that no one is willing to aid, abet or be complicit in its genocide and war crimes and illegal occupation, which all peace-loving states and peoples demand must come to an end," the message said.
Israel rejects allegations that it committed genocide and war crimes and is illegally occupying Palestinian territory.
Netanyahu at one point held up a pager while reminding the audience of September 17, 2024, when Israel remotely detonated thousands of pagers that had been distributed to the Hezbollah militant group in Lebanon.
The pager attack wounded more than 3000 people and killed 12, including two children.
He reminded the assembly of the scale of the October 7, 2023, attack in which Hamas militants killed about 1200 people.
"This is equal to 40,000 innocent Americans slaughtered in less than 24 hours," he said, or 16 times the death count on the 9/11 attacks that left nearly 3000 people dead in New York.
Netanyahu thanked US President Donald Trump for taking decisive action against Iran, and he insisted Israel would prevail: "We will continue to win because we have no other choice".
