Demonstrations sparked by a U.S.-made video resumed in Egypt and spread to Yemen, Iraq and Iran, leaving four protesters dead and threatening to escalate further in the Muslim world on Friday, the region's customary day for prayer and protest.

Two days after armed attackers killed the ambassador and three other Americans at the U.S. Consulate in Benghazi, Libya, hundreds of young men breached some outer layers of security outside the fortified U.S. Embassy in Yemen's capital.

The embassy in San'a had already been evacuated by U.S. and local staff, according to Yemeni officials, leaving Yemeni security forces to stave off protesters, who battered a guard post with pickaxes and set fire to vehicles and the embassy's American flag. Four protesters died from gunshot wounds in the clashes, and 11 were injured, while 24 security forces were injured, Yemeni officials said.

While the protests were initially triggered by a video made in the U.S. and seen as insulting to the Muslim Prophet Muhammad, there were indications on Thursday that many protesters were using the unrest to put pressure on their own countries' fledgling governments.

In San'a, demonstrators and activists said a range of opposition blocs, including an Islamist fundamentalist political party, a minority Shiite Muslim movement, and young democracy activists, had called on Wednesday for Thursday's protests.