As Prime Minister Benjamin Netanyahu of Israel approached the rostrum to address the United Nations General Assembly on Friday, representatives of nations from around the world streamed out in protest. New York City’s mayor, Eric Adams, settled in to watch him speak.

Throughout Mr. Netanyahu’s fire and brimstone address, delivered to a mostly empty hall, a group of Israel’s supporters sat in a section reserved for guests. Mr. Netanyahu’s wife, Sara Netanyahu, sat beside their son and watched her husband tear into critics of the country’s invasion of Gaza, who he said had caved “when the going got tough.”

Two rows behind them, Mr. Adams watched with a translation headset clipped to his ear, even though Mr. Netanyahu was speaking in English. The mayor met with Mr. Netanyahu briefly afterward and they smiled for photos together.

Outside, on the streets of Manhattan, thousands of demonstrators were marching toward the U.N. to express their anger toward Mr. Netanyahu over the war, which has killed more than 60,000 Palestinians, according to officials there, and left at least half a million people facing starvation.

Mr. Adams’s attendance at Mr. Netanyahu’s speech kept the Middle East at the white-hot center of a mayor’s race where the candidates hold starkly different views about the war in Gaza. The protesters in the streets mirrored the discontent voiced by Assemblyman Zohran Mamdani, the Democratic nominee and the leading candidate in the race.

In a statement responding to Mr. Netanyahu’s visit, Mr. Mamdani, who has said the prime minister should be arrested on a warrant from the International Criminal Court if he comes to New York, noted that Mr. Netanyahu’s flight this week had to “circumvent the airspace of countries which might enforce” that warrant.

Adding that Mr. Adams was set to “greet Netanyahu as a friend,” Mr. Mamdani highlighted his distance from the mayor on the war, as well as from former Gov. Andrew M. Cuomo, another electoral rival and staunch supporter of Israel.

“A mayor cannot end these atrocities,” Mr. Mamdani said. “But they can speak for the values of this city: a commitment to human rights for all people, including Palestinians, and a yearning for peace and justice. Even if those values are anathema to those in to those in power now, they will endure.”

Mr. Mamdani won his party’s primary without abandoning a series of positions on Israel that had long been seen as non-starters for candidates running for higher office in New York City. He has often described Israel’s conduct in Gaza as a genocide, an assessment shared by a U.N. commission investigating the war and other experts.

Mr. Mamdani’s views have resonated with many voters who have grown increasingly angry about Israel’s conduct in the war, which came in response to the terrorist attack by Hamas on Oct. 7, 2023, that killed about 1,200 people in Israel, according to officials there.

In his own statement on Friday, Mr. Adams said it was important to allow “everyone to speak freely” at the United Nations.

“At a time when much of the world is turning its back on the Jewish State of Israel, the mayor of the largest Jewish community outside of Israel must remain steadfast in our support for Israel, its right to defend itself, eliminate Hamas, and bring every single one of their hostages home,” Mr. Adams said. He made no mention of the Palestinians who had been killed in Gaza.

The mayor added that he had thanked Mr. Netanyahu “for defending the western world and our way of life,” a remark that drew Mr. Mamdani’s ire later on Friday.

“I can’t even begin to explain the offense that that brings to New Yorkers across the five boroughs,” Mr. Mamdani said at a news conference, in which he also said that the hostages Hamas stills holds should be freed. “How can we describe the killing of a child once an hour, every hour, for close to two years, as a defense of our way of life? That is not a way of life that any of us practice in this city.”

Politicians in New York have traditionally hugged Israel and its leaders tightly. And Mr. Adams and Mr. Cuomo, both of whom are Democrats running on independent lines, have sought to turn Mr. Mamdani’s views on Israel into a liability.

They have criticized him for labeling the war a genocide, and Mr. Adams has repeatedly called him an antisemite, an accusation Mr. Mamdani denies. (Mr. Mamdani has spoken frequently about expanding the city’s efforts to protect Jewish New Yorkers and combat antisemitism if he is elected.)

Mr. Cuomo has for years been a very public supporter of Israel. Last year, he said he would join Mr. Netanyahu’s legal defense team. More recently, Mr. Cuomo has acknowledged the barbarity of the Gaza war but declined to specify whether he believed Israel had committed war crimes, saying, “That would require a legal analysis that I haven’t done.”

Asked about the Israeli leader’s visit and whether Mr. Cuomo considered the war a genocide, Rich Azzopardi, a spokesman for Mr. Cuomo, said on Friday, “The hostages must be returned, the violence must end, and Hamas must be eliminated.”

As Mr. Cuomo and Mr. Adams continue to lag in the polls, their stances on Israel and attacks on Mr. Mamdani’s position have seemed increasingly like a political miscalculation.

A recent survey by The New York Times and Siena University found that likely voters in New York now broadly sympathize with Palestinians over Israelis in the ongoing conflict, and believe Mr. Mamdani has done the best job of all the candidates of addressing the issue.

At Friday’s demonstrations, protesters wore buttons and other apparel supporting both the Palestinian cause and Mr. Mamdani’s campaign. They criticized Mr. Netanyahu and, in some cases, Mr. Adams for supporting him.

Whitney Leigh, 56, a retired lawyer living in Bedford-Stuyvesant, Brooklyn, demonstrated in a “Hot Girls for Zohran” T-shirt that he picked up as he canvassed for Mr. Mamdani.

He described Mr. Adams as a “creature of the powerful” and said his decision to meet with Mr. Netanyahu was disappointing. Mr. Mamdani, he said, was “not beholden to the powerful interests that have run the city to the ground.”

Another protester, Alison Klemp, 38, said she believed Mr. Adams was only out for himself and that Mr. Mamdani’s success represented something hopeful for the city.

Mr. Mamdani’s views on the war showed he stood “for something that so many people agree with,” she said, even as “so many of our represented leaders refuse to listen.”

Alon Pinkas, a former Israeli diplomat to the United States, said in an interview that Mr. Netanyahu’s meeting with Mr. Adams was, more than anything else, a sign of the prime minister’s distaste for Mr. Mamdani’s politics and desire to placate President Trump.

Meeting with an opponent of Mr. Mamdani, whom Mr. Trump also dislikes, helps Mr. Netanyahu convey to the president that their worldviews are aligned, Mr. Pinkas said.

“Adams and Netanyahu have nothing in common except that they have both been indicted,” he said. “What are they going to talk about? How the F train works?”

Olivia Bensimon, Wesley Parnell and Jeffery C. Mays contributed reporting.