More protests are expected across the Middle East over an anti-Islam film that has sparked attacks on US embassies in the region.
Egypt's Muslim Brotherhood has called for a million-man march in the capital, Cairo, where police have again clashed overnight with protesters near the city's American embassy.
Demonstrations are also due to take place in Sudan, after state-backed Islamic scholars urged people to "defend Prophet Mohammad".
Salah el Din Awad, general secretary of the scholars' group in Khartoum state, said: "We have 5,000 mosques in Khartoum with two million people attending Friday prayers.
"We will all go out to defend Prophet Mohammad. We will do this peacefully but with strength."
Religious clerics in Pakistan joined calls for a day of protests, while further demonstrations are likely to continue in countries including Kuwait, Tunisia and Morocco.
The film that prompted the anger was produced in the US and depicts the Prophet Mohammed having sex, calling for massacres and as a homosexual.
Gunmen claiming to be protesting over the film fired rocket-propelled grenades at the US consulate in the Libyan city Benghazi on Tuesday, killing ambassador Christopher Stevens and three other Americans.
Eastern Libya's deputy interior minister, Wanis el Sharef, said the film protests were a cover for a violent assault planned to coincide with the anniversary of the September 11 terror attacks on the US in 2001.
Libyan authorities have made four arrests since the attack, with the US sending warships to the area and President Barack Obama promising to bring those responsible to justice.
Mr Obama has also spoken to Yemen's President Abd-Rabbu Mansour Hadi, whom he thanked for his condemnation of an attack on the US embassy in capital Sanaa, where protesters broke through the main gate, smashed windows and burned cars.
A White House statement said: "President Obama expressed appreciation for the cooperation we have received from the Yemeni government and underscored the importance of working together to ensure the security of US personnel going forward."
Meanwhile, Britain's Defence Secretary Philip Hammond has raised concerns about the potential for the unrest to spread to Afghanistan, and in a newspaper interview revealed that extra steps had been taken to strengthen security around British bases.