Protesters have stormed the US diplomatic missions in Cairo and Benghazi, killing four people including the US ambassador to Libya.
The attacks follow the release of a US-produced film, which apparently ridicules Islam and the Prophet Mohammed.
In the Libyan city of Benghazi, protesters fired automatic weapons at the US consulate and then set it on fire, bombarding the building with rockets and grenades. It is reported that local men hired to guard the premises were left hopelessly outnumbered.
A Libyan official has confirmed that the US ambassador was killed in the attack, along with three other members of staff at the consulate.
In Cairo, protesters climbed over the walls of the US embassy and hauled down the American flag, replacing it with an Islamic banner. Embassy staff had reportedly already fled.
The protests erupted after the release of a film entitled the Innocence of Muslims, directed by California-based Sam Bacile. The film allegedly insults Mohammed, although its Israeli-American director told the Wall Street Journal that the movie was political and not religious.
The movie was also being promoted by American preacher Terry Jones, who sparked worldwide riots after burning a copy of the Koran in 2010. Portions of the low-budget movie have already surfaced online.
"It is an American production, not designed to attack Muslims but to show the destructive ideology of Islam. The movie further reveals in a satirical fashion the life of Mohammed," Jones said on the 11th anniversary of the 9/11 attack.
According to Islam, any depiction of Mohammed is prohibited.
The unprecedented attacks drew an immediate rebuke from US Secretary of State Hillary Clinton.
She said: "The United States deplores any intentional effort to denigrate the religious beliefs of others. Our commitment to religious tolerance goes back to the very beginning of our nation.
"But let me be clear: There is never any justification for violent acts of this kind."