A protester wearing a mask of Iranian President Mahmoud Ahmadinejad holds up a replica of a nuclear bomb. Thousands gathered Thursday near the United Nations to protest Ahmadinejad as he spoke there. Photo by Jonathan Walczak

Surrounded by skyscrapers and a looming blue sky, a young protester climbed onto a security gate a block from the United Nations and began swishing a replica of a nuclear bomb through the air.

The man, who wore a mask of Iranian President Mahmoud Admadinejad, was one of thousands who gathered Thursday morning to protest the controversial leader as he spoke at the U.N.

Ali Safavi, a spokesman for the groups who organized the event, said their demands are simple.

“For one, telling Ahmadinejad he has no place at the United Nations representing the people of Iran,” Safavi said. “He should instead be put on trial by an international tribunal for the crimes he and his regime have perpetrated against the people of Iran.”

Protesters waved Iranian flags and held up photos of opposition candidates.

Dramatic representations of what they called Ahmadinejad’s cruelty were scattered around Dag Hammarskjold Plaza, on 47th Street and First Avenue, including a young man who stood, covered in fake blood, with a noose around his neck.

Next to the simulated hanging, a speaker boomed sounds of people screaming and in pain.

“I have a very tight connection with the political activists in Iran and also the student movement,” said Roya Teimouri, 50, who moved to the United States in 1975. “So most of them are in prison, and things don’t look good. They’re being tortured.”

Two hours into the rally, former Mayor Rudy Giuliani addressed the crowd, amid cries of “U.N., U.N., listen to this — Ahmadinejad is a terrorist.”

“All you are here today to cry out to the world to stand with you and your brothers and sisters who are oppressed by the brutal regime in Iran,” Giuliani said.

Speaking to reporters afterward, Giuliani said the United States needs to demonstrate its strength and be tough on Iran.

“The bullies take advantage of weakness,” he said. “The tyrants feed on acquiescence.”

Other speakers included former U.S. ambassador to the U.N. John Bolton.

Security was tight around the Midtown hotel where Ahmadinejad was staying, and also around the United Nations. Groups of police officers hurried gawking tourists along, and some streets were entirely blocked off.

President Barack Obama was scheduled to speak at the U.N. later in the day.

Safavi, the spokesman, said organizers wanted to emphasize how important it is for the world to support internal opposition in Iran.

“One does not need to be confined to the dichotomy of war or a tyrannical regime,” he said. “The international community, and in particular the United States, ought to look to the main opposition and the people in Iran.”

Safavi said Iranian-Americans from 35 states came for the protest.

One of them, Firouz Daneshgari, a professor at Case Western University, addressed the crowd on behalf of a group of Iranian-American professionals.

Asked later what he would tell Ahmadinejad if he could speak to him, Danesghari, who lives in Ohio, answered quickly.

“I would tell him, ‘Go to hell,’ ” Daneshgari said. “You don’t represent us. You’re a murderer, personally — through your system, you have killed thousands of people, and you don’t deserve to represent Iran.”