TURMOIL IN THE MIDDLE EAST

SANA, Yemen — Turmoil in the Arab world linked to an American-made video denigrating the Prophet Muhammad spread on Thursday to Yemen, where hundreds of protesters attacked the American Embassy, two days after assailants killed four Americans in Libya, including the ambassador, and crowds tried to overrun the embassy compound in Cairo.

News reports also spoke of a protest in Tehran, where around 500 Iranians chanting “Death to America” tried to converge on the Swiss Embassy, which handles United States interests in the absence of formal diplomatic relations with Washington. Hundreds of police officers held the crowds back from the diplomatic compound, witnesses said.

For a third consecutive day at the American Embassy in Cairo, protesters scuffled with police officers firing tear gas, witnesses said, and the state news agency reported that 13 people were injured. In Iraq, a militant Shiite group, Asaib Ahl al-Haq, once known for its violent attacks on Americans and other Westerners, reportedly said the video “will put all American interests in danger.” Protests were also reported at American missions in Morocco, Sudan and Tunisia, where the police also fired tear gas to disperse crowds. There were also news reports that the authorities in Afghanistan had suppressed Internet access to prevent users from viewing the offending video online in order to forestall protests there.

In Washington, Secretary of State Hillary Rodham Clinton criticized the violent anti-American protests but she also denounced the video in forceful terms. “This video is disgusting and reprehensible,” Mrs. Clinton said in remarks at the State Department, broadcast live on CNN. She also said, “The U.S. government had absolutely nothing to do with this video.”

The new violence came as news reports from Libya said the authorities there had made an unspecified number of arrests in connection with the killings of the four Americans in the mayhem that engulfed the United States Consulate in the eastern city of Benghazi on Tuesday.

In addition, the third of those four victims was positively identified by his family as Glen Doherty, 42, of Winchester, Mass., a former Navy SEAL working as a security officer. Mr. Doherty died along with the American ambassador to Libya. J. Christopher Stevens, and another diplomat, Sean Smith. The fourth American has not yet been identified.

In Sana, the capital of Yemen, witnesses said government security forces tried to disperse a crowd at the fortified American Embassy compound in the eastern part of the city. But protesters succeeded in breaking through an outer perimeter protecting the embassy, clambering over a high wall and setting fire to a building.

They were forced to retreat after trying to plunder furniture and computers, the witnesses said.

Security forces guarding the embassy fired into the air as protesters set two vehicles alight and burned tires. Protesters tore down and burned an American flag, replacing it with their own banner proclaiming the Islamic faith, witnesses said.

There were no immediate reports of American casualties, nor was there any indication that the protesters had managed to breach the main diplomatic buildings within the compound. Yemeni officials said a number of protesters were wounded and some were arrested, but did not provide figures. Hours after the attack started, smoke was still seen rising from the area.

By early afternoon, one witness, Yahya Yousef, who lives opposite the embassy, said: “Now almost everyone is out, and firing has ceased. We saw protesters getting out with some stuff from inside.”

The protests came hours after a Muslim cleric, Abdul Majid al- Zindani, urged followers to emulate the protests in Libya and Egypt, Sana residents said. Mr. Zindani, a onetime mentor to Osama bin Laden, was named a “specially designated global terrorist” by the United States Treasury Department in 2004.

The crowd gathered a day after the embassy warned Americans in a posting on its Web site, “In the wake of recent events in Libya and Egypt, there is the possibility of protests in Yemen, and specifically in the vicinity of the U.S. Embassy, in the coming days.”

“The U.S. Embassy continues to recommend that U.S. citizens avoid large gatherings. Even demonstrations or events intended to be peaceful can turn confrontational and possibly escalate into violence. U.S. citizens in Yemen are urged to monitor local news reports and to plan their activities accordingly,” the Web posting said.

President Abdu Rabbu Mansour Hadi of Yemen said in a statement that he “extends his sincere apologies to President Obama and to the people of the United States of America” for the attack. Mr. Hadi took office in February after his strongman predecessor, Ali Abdullah Saleh, stepped down last November following months of violent protests.

In his statement, Mr. Hadi said he had ordered an “expeditious and thorough investigation” and promised that “the perpetrators of these acts will be prosecuted to the full extent of the law.” The statement also alluded to “divisions among Yemen’s security and military forces” between supporters of the new government and of Mr. Saleh that had exacerbated tensions.

Mohammed Albasha, a spokesman for Yemen’s Embassy in Washington, said the government strongly condemned the attack.

“Given recent regional events, earlier this morning angry protesters have unfortunately flooded the security perimeter of the U.S. Embassy,” he said in a statement. “The government of Yemen will honor international obligations to ensure the safety of diplomats and will step up security presence around all foreign missions.”

With American Marines and naval vessels heading for Libya, the ferment in Yemen added to the already volatile mix of passions that have commingled with the initial exuberance of the so-called Arab Spring.

President Mohamed Morsi of Egypt was quoted on Thursday as saying the attacks on American personnel were unacceptable, in an apparent effort to defuse the tension in Egypt. Speaking in a television address while visiting Brussels, Mr. Morsi said he supported peaceful demonstrations but rejected attacks on personnel and diplomatic missions.

“What happened is unacceptable, rejected, Bloomberg News quoted Mr. Morsi as saying. “The Prophet Muhammad taught us to respect human life.” But he also warned against maligning Islam’s founding prophet. “The Prophet Muhammad and Islamic sanctities are red lines for all of us.”

Little is known about the origin of the video that provoked the protests, which is called “Innocence of Muslims.” It was made in obscurity somewhere in Southern California and promoted by a network of right-wing Christians with a history of animosity toward Muslims. When a 14-minute trailer of it was posted on YouTube in June, it was barely noticed.

But when the amateurish video was translated into Arabic and reposted twice on YouTube in the days before Sept. 11, and promoted by leaders of the Coptic diaspora in the United States, it drew nearly one million views and set off bloody demonstrations.

Many Muslims object strongly to any representation of Muhammad, and, as in the case of cartoons lampooning Islam’s founder published in a Danish newspaper in 2005, are particularly enraged by negative depictions. The contentious video shows the prophet as a villainous homosexual and child-molesting buffoon.

The initial eruption of protests in Egypt and Libya came on the anniversary of the Sept. 11, 2001, attacks in the United States.

Referring to the killings in Benghazi, Libya, American and European officials said on Wednesday that while many details about the attack remained unclear, the assailants seemed organized, well trained and heavily armed and appeared to have at least some level of advance planning. But the officials cautioned that it was too soon to tell whether the attack was related to the Sept. 11 commemoration.

On Tuesday, a car bomb exploded in Yemen alongside a convoy of vehicles used by Yemen’s defense minister, killing seven bodyguards and five civilians in the heart of the capital, while the minister escaped unharmed, government and hospital officials said. The attack came one day after a top operative of Al Qaeda in Yemen was killed in what Yemeni officials called an American drone strike.

Those episodes and the violence on Thursday spoke to the continued volatility in poverty-stricken Yemen, where the United States is seeking to eradicate militant cells held responsible for a number of conspiracies, including an attempt by an operative of Al Qaeda to detonate a bomb hidden in his clothes on a flight bound for Detroit in December 2009.

The blast on Tuesday in downtown Sana tore through a thoroughfare between the cabinet office and the state radio building, shattering buildings and wrecking a vehicle carrying the seven bodyguards, seconds after the minister himself, Maj. Gen. Mohammed Nasser Ahmed, had passed by in another vehicle after a weekly cabinet meeting.

Nasser Arrabyee reported from Sana, Yemen, and Alan Cowell from London. Reporting was contributed by Ramtin Rastin from Tehran; David D. Kirkpatrick from Cairo; Kareem Fahim from Beirut, Lebanon; Jess Bidgood from Boston; and Rick Gladstone from New York.