CAIRO, Egypt (CNN) -- Angry protesters climbed the walls of the U.S. Embassy in Cairo on Tuesday and tore down the American flag, apparently in protest of a film thought to insult the Prophet Mohammed.
A volley of warning shots were fired as a large crowd gathered around the compound, said CNN producer Mohammed Fahmy, who was on the scene, though it is not clear who fired the shots.
Egyptian police and army personnel have since formed defensive lines around the facility in an effort to prevent the demonstrators from advancing farther, but not before the protesters affixed their standard atop the embassy.
The black flag, which hangs atop a ladder inside the compound, is adorned with white characters that read, "There is no God but Allah and Mohammad is his messenger," an emblem often used in al Qaeda propaganda. It was unclear which film upset the protesters.
Others in the crowd expressed their grievances over U.S. policy, chanting anti-American slogans and holding up bits of a shredded American flag to television camera crews. An embassy operator told CNN that the compound had been cleared of diplomatic personnel earlier Tuesday, ahead of the apparent threat, while Egyptian riot police and the army were called in.
Several individuals claimed responsibility for organizing the demonstrations, including Salafist leader Wesam Abdel-Wareth, who is president of Egypt's conservative Hekma television channel.
Mohammed al-Zawahiri -- the brother of al Qaeda's leader Ayman al-Zawahiri -- added that "we called for the peaceful protest joined by different Islamic factions including the Islamic Jihad, Hazem Abu Ismael movement."
"We were surprised to see the big numbers show up including the soccer Ultra fans," he said. "I just want to say, how would the Americans feel if films insulting leading Christian figures like the pope or historical figures like Abraham Lincoln were produced?"
He added that "the film portrays the prophet in a very ugly manner, eluding to topics like sex, which is not acceptable."
U.S. embassy officials issued a warning to Americans in Egypt, telling them to avoid the demonstrations which "may gather in front of the U.S. Embassy, or Egyptian government buildings such as the People's Assembly and Ministry of Interior."
"It is unclear if large numbers will take to the streets, but clashes may occur should two opposing groups come into contact with one another," the embassy said in a statement. "Large gatherings and non-essential travel in and around Downtown and Garden City should be avoided this afternoon."
The U.S. Embassy said that it "condemns the continuing efforts by misguided individuals to hurt the religious feelings of Muslims -- as we condemn efforts to offend believers of all religions."
"Respect for religious beliefs is a cornerstone of American democracy," the statement said. "We firmly reject the actions by those who abuse the universal right of free speech to hurt the religious beliefs of others."
The incident occurred on the 11th anniversary of the 9/11 attacks as crowds gathered in somber remembrance of the nearly 3,000 people killed that day.
In 2008, an anti-Muslim and anti-immigration parliamentarian from the Netherlands sparked international outcry when he produced a film that portrayed Islam as a violent religion.
Geert Wilders' film "Fitna," which he released online, featured images of terrorist acts superimposed over verses from the Quran.