The authorities in Pakistan have called on the army to control protesters demonstrating against an amateur video mocking Islam outside the US embassy in the capital, Islamabad.
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.
Chaotic scenes
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."
A demonstration in the same area on Wednesday saw around 500 protesters gather outside the gates of the enclave.
The US State Department earlier issued a warning against any non-essential travel to Pakistan.
They also "strongly urged" US citizens in Pakistan to avoid protests and large gatherings.
Anti-US sentiment has been growing since people became aware of the amateur film, Innocence of Muslims, earlier this month.
The US Ambassador to Libya was killed in an attack on the US consulate in Benghazi, on 11 September.
Protests in countries around the world then took place.
Tensions with the West have been further inflamed by the publication by a French magazine of obscene cartoons of the Prophet Muhammad on Wednesday.
The Pakistani government has called a national holiday on Friday to enable people to demonstrate peacefully.
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