On the fourth day of demonstrations in Egypt, protesters threw rocks and Molotov cocktails at the U.S. Embassy. Police used tear gas in a bid to disperse the crowd, The New York Times reported.
Witnesses in Tunisia said a U.S.-run school was burned and looted, while in Sudan rage was directed at other missions as well.
In Nigeria, about 2,000 people assembled outside a mosque in the city of Jos, CNN reported. Police fired warning shots in an effort to break up the crowd.
There were many smaller demonstrations and riots, the BBC said. In London, about 200 people joined a non-violent protest outside the U.S. Embassy, while a crowd of about 1,000 burned U.S. President Barack Obama in effigy in Jalalabad, Afghanistan.
Obama and Secretary of State Hillary Clinton were on hand at Joint Base Andrews Naval Air Facility, Md., Friday as the bodies of U.S. Ambassador Christopher Stevens and three other U.S. diplomats killed Tuesday in Libya arrived in the United States.
During a brief ceremony, Obama said Stevens was "everything America could want in an ambassador" and promised the sacrifice of the U.S. diplomats "will never be forgotten."
"We will bring to justice those who took them from us," he said. "We will stand fast against the violence on our diplomatic missions.
"Most of all, even in our grief, we will be resolute," the president said. "Because we are Americans, we will hold our heads high.
"The United States of America will never retreat from the world."
Clinton called on leaders in the Middle East to deal with violence in their countries, saying people in those nations "did not trade the tyranny of a dictator for the tyranny of a mob."
"We will wipe away our tears, stiffen our spine and face the future undaunted," she said.
The violence began Tuesday, the 11th anniversary of the Sept. 11, 2001, terror attacks on New York and Washington. The protests were ostensibly sparked by a 14-minute trailer for "Innocence of Muslims" posted on YouTube depicting the Prophet Muhammad as a power- and sex-crazed bisexual pedophile.
In Cairo, where more than 200 protesters have been injured since Wednesday, hundreds of demonstrators filled Tahrir Square as well as an area outside the U.S. Embassy in the city's Garden City area following midday prayers, Ahram Online reported.
"If you love the prophet, stay peaceful," Imam Mazhar Shaheen exhorted the crowd.
However, Ahram Online said Salafist protesters, waving black flags, demanded action.
What were described as "limited clashes" were reported outside the embassy grounds and the Semiramis InterContental Hotel.
Egyptian soldiers began building a wall to protect the embassy.
Late in the day, the Muslim Brotherhood organized a demonstration to show unity between Muslims and Christians, al-Masry al-Youm reported.
In Libya, where Stevens and three other U.S. diplomats were killed Tuesday, the Shariah Supporters Battalion denied it was responsible for the deadly attack on the U.S. Consulate in Benghazi, The Tripoli Post reported.
The Libyan government said Thursday it arrested four people in the consulate attack as Libyan and U.S. officials mounted a manhunt for others believed to have been involved.
Shouts of "Death to America" and "We condemn the film" rang through Nangarhar province in Afghanistan Friday where demonstrators burned a U.S. flag.
The protests spread to other countries' embassies, including the German and British embassies in Khartoum, Sudan, CNN reported. At the German Embassy, demonstrators replaced the German flag with an Islamist banner. Military sources told CNN late Friday a team of U.S. Marines was sent to Khartoum to protect U.S. posts there, with similar teams going to Libya and Yemen.
Vice President Joe Biden called Sudanese Vice President Ali Osman Taha Friday to express concern for the security of the U.S. Embassy and other foreign missions in Khartoum, the White House said in a statement. Biden reminded Taha his government is responsible for protecting diplomats and diplomatic facilities and said the United States' top priority is the safety of its diplomatic presence abroad.
The Times quoted German Foreign Minister Guido Westerwelle as saying part of the German Embassy in Khartoum had been "stormed" and "set aflame" but all personnel were safe.
Security personnel were in place at U.S. diplomatic posts throughout the Middle East, an unidentified senior Obama administration official told CNN.
"We are in a full-court press at every single one of the posts in the Middle East and anywhere else there is any chance of demonstrations after Friday services to make sure nothing bad happens -- and to have the security in place in case bad things do happen," the official said Thursday.
Yemeni security forces, backed by water cannons, blocked streets near the U.S. Embassy Friday as several dozen protesters gathered near the mission, carrying posters and shouting slogans. CNN reported witnesses said security forces opened fire on protesters but it wasn't clear what type of weapons were used. Two protesters died in demonstrations there Thursday. The Pentagon sent 50 marines to Sanaa to secure U.S. interests, the Times said.
Iraqi protesters denounced the United States and Israel, and called for the expulsion of U.S. diplomats and an apology for the film.
"We want the U.S. government to prove that there is justice by stopping this movie and punishing the director and his staff," the Times quoted Sheik Ahmad al-A'ani as saying in Baghdad.
Elsewhere in Iraq, U.S. and Israeli flags were burned as some preachers expressed disbelief Stevens and the others had been killed in an attack and instead accused the United States of orchestrating the killings as a pretext for an attack, the Times said.
In Gaza, demonstrators gathered outside Parliament where the crowd stomped on U.S. and Israeli flags, the Times said.
Egyptian leaders have been working to repair diplomatic strains with the United States created by their initial response to Tuesday's assault on the U.S. Embassy, when they focused more on anti-American opinion than on condemning the violence, the Times said.
During a phone call Wednesday, Obama warned Egyptian President Mohamed Morsi U.S.-Egyptian relations could be in danger if Cairo failed to protect U.S. diplomats and respond more firmly against the attacks.
The Organization of Islamic Cooperation has condemned the violent acts on U.S. missions in Cairo and Benghazi, with its secretary-general saying such actions could not be condoned under any circumstances, Khaama Press reported.
Speaking from Jeddah, Saudi Arabia, OIC Secretary-General Ekmeleddin Ihsanoglu said the anti-Islam film was deplorable but reacting with violence cannot be accepted.
Ihsanoglu called for restraint and urged law enforcement officials to do what was necessary to restore calm.
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