Addressing a crowd of Holocaust survivors and their families, Israeli and American diplomats and politicians yesterday warned of a 21st century holocaust if Iran’s nuclear ambitions are not thwarted.

The American ambassador to the United Nations, John Bolton, Senator Schumer, and the consul general of Israel, Arye Mekel, each pledged a commitment to keeping nuclear weapons out of the hands of the president of Iran, Mahmoud Ahmadinejad, who denies the Holocaust and has repeatedly expressed his desire for Israel’s destruction.

“This is not an Israeli issue,” Mr. Mekel told more than 1,000 attendees at a Holocaust Remembrance Day event at Hunter College. “This is a world problem. The world must stop Iran.” The event was sponsored by the Museum of Jewish Heritage – A Living Memorial to the Holocaust, the Warsaw Ghetto Resistance Organization, and other American and Israeli groups.

Holocaust Remembrance Day, or Yom HaShoah, is observed worldwide tomorrow.

Mr. Mekel urged the U.N. to impose economic and political sanctions against Iran. “Let’s be clear,” he said. “That country cannot, cannot have nuclear weapons.”

Stressing the contemporary relevance of Holocaust Remembrance Day, Mr. Bolton said the potential for a “nuclear holocaust” looms as Iran pursues its nuclear program, despite international pressure.

“Holocaust remembrance is not simply for the past, but for the present and the future as well,” Mr. Bolton said. “I’m honored to be part of that struggle.”

With its frequent condemnations of Israel, the U.N. has “disappointed the ideals” of those who established the transnational body after World War II, Mr. Bolton said.

Mr. Schumer also criticized Mr. Ahmadinejad, calling him a “madman,” but said problems of Holocaust denial and anti-Semitism go further. “In Europe itself, we see a vehement double standard against Israel and the Jewish people,” he said. “We see countries recognizing a government, Hamas, dedicated to killing Jewish women, children, and men.”

Mr. Schumer compared Hamas, the terrorist organization that recently took control of the Palestinian Authority, with the Nazis. He said that although Hamas employs different methods of killing from the Nazis, the group seeks the same result as Hitler.

Norbert Friedman, an 84-year-old Krakow native who survived 11 concentration and death camps, said yesterday that promises to prevent future holocausts are “only empty words unless they’re followed up with deeds.”

Mr. Friedman referred to the 1994 genocide in Rwanda, which took the lives of up to a million people, and the mass murders now taking place in the Darfur region of Sudan. “The cry of a mother in Darfur is as painful as the cry of my own mother,” he said. “We cannot disregard that.”

At the event dozens of survivors, including the sex therapist Dr. Ruth Westheimer, along with their children and grandchildren lit memorial candles in honor of those who died during the Holocaust.

“Although all of us have come here to be with each other, in some paradoxical way we each have come here to be alone with our thoughts and prayers and memories,” the director of the Museum of Jewish Heritage, David Marwell, said.

Another survivor, Samuel Bloch, addressed the crowd in Yiddish, the mother tongue of many of Eastern European Jews, which is no longer widely spoken. During his speech, in which he spoke of recognizing injustice, Mr. Bloch uttered only one word in English: “Remember.”