Angry protesters climbed the walls of the U.S. Embassy in Cairo on Tuesday and hauled down the American flag, replacing it with a black flag with Islamic emblems.
The incident prompted U.S. Embassy guards to fire off a volley of warning shots as a large crowd gathered outside, apparently upset about the production of a film thought to insult the Prophet Mohammed, said CNN producer Mohammed Fahmy, who was on the scene.
It is not clear which film upset the protesters.
Others expressed more general grievances about U.S. policy, chanting anti-American slogans and holding up bits of a shredded American flag.
An embassy operator told CNN that the facility had been cleared of diplomatic personnel earlier Tuesday, ahead of the apparent threat, while Egyptian riot police were called to help secure the area.
The U.S. Embassy said in a statement 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 September 11 attacks in New York, Washington and Pennsylvania, as crowds gathered in somber remembrance of those killed.