Live rounds have been heard and tear gas has been fired as police struggled to contain the crowd of thousands.
The film, which was made in the US, is said to insult the Prophet Muhammad.
Several people have died in protests in the past week against the film in the Middle East and North Africa.
The BBC’s Aleem Maqbool in Islamabad says some protesters have said they will not leave the diplomatic enclave until the US embassy is on fire.
However, he has not yet seen any evidence of the army’s arrival.
Streets leading to the enclave, where most of the embassies are housed, were earlier blocked off by shipping containers in an effort to increase security.
Television pictures have shown chaotic scenes as police tried to gain control of the situation.
Effigies have been burned, and stones and other missiles have been thrown at police.
One demonstrator told Reuters: “Our policemen are not any better than the Americans, because they are trying to stop us.”
Read the rest at The BBC