Transcript for American Embassies Under Siege Across Middle East

Tonight, it was a day of fury, as violent anti-american protests erupt at u.S. Embassies across the arab world. In cairo tonight, clashes with police have turned violent, after hundreds stormed the american compound in yemen earlier today. It's all part of the uproar over an american-made movie that mocks the muslim prophet muhammad. Tonight, american officials are warning that extremist groups could spread this violence back here at home and abc's martha raddatz brings us the latest. Reporter: Tonight, protesters and egyptian security forces are facing off outside the embassy in cairo. The crowds, throwing stones and molotov cocktails, greeted with tear gas in return. It has been a long day of unrest in the region. This morning, outside the u.S. Embassy in yemen's capital, protesters smashing windows and climbing the walls to the compound. At around 4:30 a.M. East coast time, yemen's spokespe in washington tweeted in all caps, "protesters have stormed the u.S. Embassy in sana'a." The assault, only the latest of a growing number fueled by anger at an amateurish, low budget film made in los angeles and promoted the koran-burning gas or the, terry jones of florida. There must be an action. Reporter: The film is clearly meant to denigrate the muslim prophet muhammad, depicting him as a child molester. There's really no possibility of maintaining a lid on the kind of provocative material like this film from getting out into the region. Reporter: For three days now, protests have been raging across the arab world. It started on tuesday, the anniversary of 9/11, when a group scaled the walls of the u.S. Embassy in cairo, tore down the stars and stripes and raised black flags reading "there is no god but allah." By tuesday night, the violence was spreading and deepening. The american consulate in benghazi, libya, was in flames. And the ambassador and three other americans, dead, after militants attacked with gun fire and rocket-propelled grenades. By wednesday, the crowd outside the embassy in cairo had grown larger and the mood, ugly. The rhetoric now, a world away from the heady days of the arab spring last year. When I visited yemen during its own uprising, the atmosphere was optimistic. Things have clearly changed. We do have to recognize that this kind of outrage that we're seeing in the region is a result of an enormous amount of frustration with the lack of fulfilled promises about the arab spring. The arab spring wasn't just about freedom and democracy. It was about social and economic progress. Reporter: Despite the death and utter devastation at the u.S. Mission in benghazi, the state department insists there was a robust security presence. We determined that the security at benghazi was appropriate for what we knew. Reporter: Yet, just the day before the attack, al qaeda's leader urged libyans to retaliate against americans for the death of a libyan-born militant, killed by u.S. Drones. And days prior to the attack, the state department knew that the anti-muslim movie that has caused outrage, had been running on egyptian television. Yet, there were no warnings about it to other u.S. Missions. That film is now the focal point of the anti-american sentiment roaring through the middle east. This video is disgusting and reprehensible. There is no justification. None at all, for resing to this video, with violence. Reporter: The film's origins are still murky, but law enforcement says they've identified at least one man involved in its production, nakoula basseley nakoula. A coptic christian who lives in california. Today, media and place were gathered outside his suburban home, but he never appeared. Sources tell abc ne b kr nbc news he has received threatening calls and is scared for his life. Tonight, protesters are still gathered outside the u.S. Embassy in egypt, whose government, led by the muslim brotherhood, is under fire for not con telling the attacks quickly or forcibly enough. I don't think that we could consider them an ally, but we don't consider them an enemy. Reporter: In an interview with tell money doe, president obama was surprisingly stern about the egyptian response. And if they take action that indicate they're not taking those responsibilities, as all other countries do where we have embassies, I think that's going to be a real big problem. Reporter: And the u.S. Embassy in cairo pointed out that while the muslim brotherhood's english twitter feed pledged support for the u.S., Its arabic language field was focused on decrying the film. "Have you checked our your own arabic fields? I hope you know we read those, too." The protests will likely continue into friday, the muslim holy day, and u.S. Embassies around the world remain on high alert. The aftermath of a low budget film with horrible reviews, but a huge and angry audience. For "nightline," I'm martha raddatz in washington.

This transcript has been automatically generated and may not be 100% accurate.