Protesters have breached the wall of the American embassy in Cairo and torn down a flag over a US-made film which they say is insulting to the Prophet Muhammad.
The American flag, which was flying at half mast to mark the 9/11 attacks, was replaced by an Islamist banner.
Thousands of protesters had gathered outside the embassy.
Among the film's producers is said to be a pastor who burnt copies of the Koran earlier this year.
Among the protesters outside the embassy were hardline Islamists known as Salafists and also members of a football supporters' club known as Ultras.
They say the film is about to be shown in the US.
On Tuesday night, a handful of protesters continued to sit on the wall of the embassy but the compound was surrounded by Egyptian riot police and there was no sign of any confrontation, says the BBC's Jon Leyne at the scene in Cairo.
A spokesman for the US embassy in Cairo has categorically denied that any shots had been fired at any time during the protest.
The US embassy earlier issued a statement condemning "the 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."
The US state department said it was working with Egyptian security to try to restore order at the embassy and to get the situation under control.
Spokeswoman Victoria Nuland said there were no reports of injuries, adding: "We had some people breach the wall, take the flag down and replace it.
"What I heard was that it was replaced with a... plain black flag. But I may not be correct in that."
The film which sparked the protest is said to have been produced by US pastor Terry Jones and co-produced by some Egyptian Copt expatriates.
Egyptian protesters condemned what they said was the humiliation of the Prophet of Islam under the pretext of freedom of speech.