Demonstrators incensed by a film that insults the Prophet Muhammad have staged protests across the Middle East.
Violent scenes have been witnessed in Egypt, Yemen, Sudan and Tunisia.
Rallies were also held in Afghanistan, Malaysia, Bangladesh, Pakistan, Iran and Iraq.
Germany’s Foreign Minister Guido Westerwelle said he understood the outrage in the Islamic world over the film but said it was no excuse for violence.
At least four people - all protesters - were killed and dozens were wounded in the demonstrations in more than 20 countries from the Middle East to Southeast Asia.
Most were peaceful but they turned violent in several nations, presenting challenges for the leaders who came to power in the Arab Spring.
Security forces worked to rein in the anti-American crowds but appeared to struggle in doing so.
Police in Cairo prevented stone-throwing protesters from getting near the US Embassy, firing tear gas and deploying armoured vehicles in a fourth day of clashes in the Egyptian capital.
One person died there after being shot by rubber bullets.
US President Barack Obama said Washington would "stand fast" against attacks on US embassies around the world.
He spoke at a somber ceremony paying tribute to four Americans -- including US Ambassador Chris Stevens - killed earlier this week when the US Consulate in Benghazi, Libya, was stormed by militants who may have used protests of the anti-Muslim film to stage an assault on the 11th anniversary of the September 11 terrorist attacks.
An elite Marine rapid response team arrived in Yemen's capital of Saana, where local security forces shot live rounds in the air and fired tear gas at a crowd of an estimated 2,000 protesters who were kept about a block away from the US Embassy, which protesters broke into the day before.
In east Jerusalem, Israeli police stopped a crowd of about 400 Palestinians from marching on the US Consulate to protest the film. Demonstrators threw bottles and stones at police, who responded by firing stun grenades. Four protesters were arrested.
Secretary of State Hillary Rodham Clinton had tried to pre-empt the violence a day earlier by saying the rage and violence aimed at American diplomatic missions was prompted by "an awful Internet video that we had nothing to do with".
Egyptian President Mohammed Morsi went on national TV and appealed to Muslims not to attack embassies. It was his first public move to restrain protesters after days of near silence and appeared aimed at easing tensions with the United States.
But the demonstrators came out after weekly Friday prayers. Many clerics in their mosque sermons urged congregations to defend their faith, denouncing the obscure movie "Innocence of Muslims" that was produced in the United States that denigrated the Prophet Muhammad.
In addition to countries where protests have occurred, US embassies around the world, including in France and Austria, issued alerts Friday advising Americans to review their personal security measures and warning them that demonstrations may occur and may turn violent.
Other embassies issuing alerts were those in Denmark, Mauritania and India. More than 50 US embassies and consulates had released similar alerts Thursday.
Several thousand people battled with Tunisian security forces outside the US Embassy in Tunis. Protesters rained stones on police firing tear gas and shooting into the air.
Some protesters scaled the embassy wall and stood on top of it, planting the Islamist flag that has become a symbol of the wave of protests: A black banner with the Islamic profession of faith, "There is no god but Allah and Muhammad is his prophet."
Police chased them off the wall and took the flag down. Two protesters were killed and 29 people were wounded, including police.
Protesters also set fire to an American school adjacent to the embassy compound and prevented firefighters from approaching it. Thick columns of black smoke wafted through the neighbourhood.
The heaviest violence came in Khartoum, Sudan, where a prominent sheik on state radio urged protesters to march on the German Embassy to protest alleged anti-Muslim graffiti on mosques in Berlin and then to the US Embassy to protest the film.
"America has long been an enemy to Islam and to Sudan," Sheik Mohammeizouly said.
Soon after, several hundred Sudanese stormed into the German Embassy, setting part of a building aflame along with trash bins and a car.
Protesters celebrated around the burning barrels as black smoke billowed into the sky until police firing tear gas drove them out of the compound.
Some then began to demonstrate outside the neighbouring British Embassy.
Several thousand then moved via a convoy of buses to the US Embassy on Khartoum's outskirts. They clashed with Sudanese police, who fired on some who tried to scale the compound's wall. It was not clear whether any protesters made it onto the embassy grounds.
The police then dispersed the crowd with tear gas, starting a stampede. Witnesses reported seeing three protesters motionless on the ground, although there was no immediate word whether they were dead or alive.
US Vice President Joe Biden called his Sudanese counterpart on Friday to express concern over the safety of US and other diplomatic missions in Khartoum, the White House said.
Mr Biden's call to Sudanese Vice President Ali Osman Taha came amid an outbreak of anti-American protests in the Middle East over a film that insults Islam.
"Vice President Biden reaffirmed the responsibility of the government of Sudan to protect diplomatic facilities and stressed the need for the Government of Sudan to ensure the protection of diplomats in Khartoum," the White House said.
Islamic militants waving black banners and shouting "God is great!" stormed an international peacekeepers' base in Egypt's Sinai and battled troops, wounding four Colombians, said a senior official with the multinational force.
The base near the border with Gaza and Israel houses some 1,500 members of the force, including US troops.
The official, speaking on condition of anonymity because he was not authorized to talk to the press, said it appeared the attack was connected to the wider protests in the region.
One protester was killed in the northern Lebanese city of Tripoli in clashes with security forces after a crowd set fire to a KFC and a Hardee's restaurant. Protesters hurled stones and glass at police in a furious melee that left 25 people injured, 18 of them police.
In his bid to head off the violence, Egypt's Morsi said "it is required by our religion to protect our guests and their homes and places of work."
He called the killing of US Ambassador Chris Stevens in Libya unacceptable in Islam. "To God, attacking a person is bigger than an attack on the Kaaba," he said, referring to Islam's holiest site in Mecca.
Mr Morsi's speech came after Obama spoke with him by telephone.
The Obama administration has been angered by Morsi's slow response to the attack Tuesday night on the US Embassy in Cairo, and the Egyptian made little more than vague statements about it for days without an outright condemnation of the security breach, in which police did nothing to stop protesters from climbing the embassy walls.
His silence reflected the heavy pressure that Morsi, a longtime figure from the Muslim Brotherhood, faces from Egypt's powerful ultraconservative Islamists. They are using the film issue to boost their own political prominence while challenging Morsi's religious credentials.
Leaders of Egypt's Jihad group, a former militant organization, held a conference in the Egyptian city of Alexandria and said anyone involved in "defamation" of the prophet should be killed. They called on Morsi to cut relations with US.
Several hundred people, mainly ultraconservatives, protested in Cairo's Tahrir Square and tore up an American flag. A firebrand ultraconservative Salafi cleric lambasted the film in his sermon, saying Muslims must defend Islam and its prophet.
"With our soul, our blood, we will avenge you, our Prophet," they chanted as police fired volleys of tear gas.
Soldiers opened fire to drive away young Muslims in the central Nigerian city of Jos, witnesses and authorities said, and demonstrators in the county's Muslim north burned a US flag.
Hundreds of hard-line Muslims held peaceful protests against the film throughout Pakistan, shouting slogans and carrying banners criticizing the US and those involved in the film.
Police in Islamabad set up barricades and razor wire to prevent protesters from getting to the diplomatic enclave, where the US Embassy and many other foreign missions are located.
About 1,500 protesters in Afghanistan's eastern city of Jalalabad shouted "Death to America" and urged President Hamid Karzai to cut relations with the US.