Demonstrators stormed the U.S. embassy in Yemen on Thursday, leading to clashes in which four people were killed, while protesters stoned Washington's mission in Cairo as anger spread over a US-produced film mocking Islam.
A security official said 34 people were also wounded in the clashes "that lasted from morning until late in the evening," and that eight were in serious condition.
Witnesses said police fired live rounds and used tear gas to disperse hundreds of protesters while troops were deployed on rooftops surrounding the mission in Sanaa.
Earlier in the day police used water cannons and fired warning shots to expel protesters who breached the perimeter wall.
One demonstrator was shot dead outside the compound as police battled to prevent any new incursion, with three others killed in successive clashes.
The protests came as U.S. and Libyan officials said they were probing a mob attack on the consulate in Benghazi that killed U.S. ambassador Christopher Stevens and three other U.S. officials on Tuesday, amid growing speculation it was the work of jihadist militants rather than just demonstrators.
As Libya announced it made arrests, U.S. Attorney-General Eric Holder told an Arab forum in Qatar, "The FBI has opened an investigation" into the deaths and the attack on the consulate.
Yemeni President Abdrabuh Mansur Hadi apologized to his U.S. counterpart President Barack Obama and the American people for the acts of a "mob" and ordered a probe.
"Those who are behind (the attack) are a mob that are not aware of the far-reaching plots of Zionist forces, especially those who made a film insulting the Prophet," Mr. Hadi said.
Some protesters reported seeing vehicles being torched by some of the demonstrators after they gained access to the compound through an unguarded security gate.
After being evicted from the complex on their first assault, protesters retreated about 100 metres from the gate, gathering near a checkpoint where they chanted anti-Jewish slogans.
Chanting "O, messenger of Allah... O, Mohammed," the protesters then launched a second bid to access the compound, prompting police to fire on the crowd, witnesses said. Clashes continued until late in the evening.
European Union foreign policy chief Catherine Ashton condemned the violence and urged Yemeni authorities to boost security at E.U. diplomatic missions in Sanaa.
"We have urged the Yemeni authorities to reinforce security of E.U. missions in Sanaa and to take the necessary measures to protect diplomats," Ms. Ashton added.
Violence also rocked the Egyptian capital Cairo, where police fired tear gas to disperse protests outside the embassy.
A total of 224 people were injured in the protest, the Egyptian health ministry said.