Hundreds of protesters gathered in front of the embassy, alternating religious slogans with chants against the US or President Obama. Some climbed over the wall and tore down the American flag, raising instead a black flag with an Islamic inscription that is often associated with Islamic extremists.
Some in the crowd were ultraconservative Islamists known as Salafis. Others were simply Muslims who were upset that their prophet had apparently been maligned in the US. Several Christians joined the crowd to protest the reported insults to Muhammad.
"These insults are not just against the prophet Muhammad, peace be upon him, but against the whole Muslim community," says teacher Abdullah Ibrahim Mohamed, who explains he heard about the film from media and Facebook. He has not seen the film but heard that it "showed the prophet as a very bad man."
"We want to send Americans a message that they should respect our religion, and respect our prophet. We do no harm to them," he says.
Angry demonstrators have periodically targeted US installations abroad over perceived insults to Islam, particularly following the deepened US involvement in the Muslim world after Sept. 11, 2001. Such protests have proven challenging to American officials, who must balance US rights to free speech with a desire for greater cooperation in the region.
Trying to strike that balance, the embassy in Cairo issued a statement earlier today in expectation of the protest condemning "continuing efforts by misguided individuals to hurt the religious feelings of Muslims as we condemn efforts to offend believers of all religions."
The statement added: "We firmly reject the actions by those who abuse the universal right of free speech to hurt the religious beliefs of others."
After news of the protests emerged, the embassy statement got passed around Twitter, generating some angry responses. "Why on earth is @USEmbassyCairo condemning not the people ripping up an American flag, but an American exercising his freedom of speech?" asked Joshua Faust, a fellow at the American Security Project in Washington.
Nader Bakkar, the spokesman for the Salafi party Nour, says the US embassy had asked him to negotiate with the protesters, and ask them not to breach the embassy walls or use violence.
"Insulting [the prophet] is like insulting every Muslim citizen across the world," he says. "We cannot accept that this is a kind of freedom" in the US.
He says he demanded that Egypt withdraw citizenship from anyone who worked on the film. He also had demands for the US to "stop these actions, and understand that [insulting the prophet] is a special case."
"There is a new situation after the revolution in Egypt," he says. "We cannot accept this kind of insulting. The relationship between the US and Egypt could be affected by this kind of attitude."
Most of the protesters said they had never seen the film, but heard about it from Egyptian television channels, newspapers, or Facebook. One religious channel broadcast what it said was a clip from a film produced in the US by Egyptian Christians who had emigrated there.
Some protesters said they heard the film was supposed to be broadcast in the US today. They said the protest was not related to the anniversary of 9/11.