Main ultraconservative Islamist protesters climbed the walls of the U.S. Embassy in Egypt's capital Tuesday and brought down the flag, replacing it with a black flag with an Islamic inscription to protest a video attacking Islam's prophet, Muhammad.

Hundreds of protesters marched to the embassy in downtown Cairo, gathering outside its walls and chanting against the movie, which was reportedly produced in the United States.

"Say it, don't fear: Their ambassador must leave," the crowd chanted.

Dozens of protesters then scaled the embassy walls, went into the courtyard and took down the flag from a pole. They brought it back to the crowd outside, which tried to burn it, but failing that, tore it apart. The protesters on the wall then raised on the flagpole a black flag with the Muslim declaration of faith on it, "There is no god but God and Muhammad is his prophet."

The flag, similar to the banner used by al-Qaida, is commonly used by ultraconservatives around the region. Almost all the embassy staff had left the compound before the protest, and the ambassador was out of town.

The protest was sparked by outrage over a video being promoted by an extreme anti-Muslim Egyptian Christian campaigner in the U.S., clips of which are available on the social website YouTube and dubbed in Egyptian Arabic. The video depicts Muhammad as a fraud, showing him having sex and calling for massacres. Muslims find it offensive to depict Muhammad in any fashion, much less in an insulting way.

By evening, the protest grew with thousands standing outside the embassy, chanting "Islamic, Islamic. The right of our prophet will not die." A group of women in black veils and robes that left only their eyes exposed chanted, "Worshippers of the Cross, leave the Prophet Muhammad alone."

Dozens of riot police lined up along the embassy walls. They did not stop protesters who continued to climb up the wall and stand on it, chanting. But it appeared they were no longer going into the embassy compound.

One young member of the ultraconservative Salafi movement, Abdel-Hamid Ibrahim said, "This is a very simple reaction to harming our prophet."

In Washington, State Department spokeswoman Victoria Nuland said the U.S. was working with Egyptian authorities to try to restore order.

Only a few staff members were still inside, as embassy security had sent most staff home early after learning of the upcoming protest, a U.S. official said, speaking on condition of anonymity because he wasn't authorized to speak publicly on the matter.

It was not exactly clear who made the video that angered the protesters.

Morris Sadek, an Egyptian-born Christian in the U.S. known for his anti-Islam views, told The Associated Press from Washington that he was promoting the video on his website and on certain TV stations, which he did not identify.

He said the video "explains the problems of the Copts who suffer from Muslims," which he blamed on the Quran itself.

For several days, Egyptian media have been reporting on the video, playing some excerpts from it and blaming Sadek for it, with ultraconservative clerics going on air to denounce it.

Medhat Klada, a representative of Coptic Christian organizations in Europe, said Sadek's views are not representative of expatriate Copts.

"He is an extremist ... We don't go down this road. He has incited the people (in Egypt) against Copts," he said, speaking from Switzerland. "We refuse any attacks on religions because of a moral position."

But he said he was concerned about the backlash from angry Islamists. "They don't know dialogue and they think that Islam will be offended from a movie."

The embassy is located in a diplomatic area in Garden city, where the British and Italian embassies are located, only a few blocks away from Tahrir Square, the center of last year's uprising that led to the ouster of Hosni Mubarak. The U.S. Embassy is built like a fortress, with a wall several meters (yards) high. But security has been scaled back in recent months, with several roadblocks leading to the facility removed after legal court cases by residents complaining their access to nearby streets was blocked.

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AP writer Bradley Klapper in Washington contributed to this report.