What began as a peaceful protest movement demanding the departure of a president is turning into a bloody battle on the streets of the Yemeni capital, pitting two powerful families against each other and raising fears of a civil war.

At their core, the increasingly violent clashes are an old-fashioned struggle for power and money between two segments of Yemen’s elite — supporters of President Ali Abdullah Saleh and tribesmen loyal to one of the country’s leading families, the Ahmars.

The fighting in Sanaa has claimed about 130 lives in 10 days and added to concerns about a breakdown in authority in the impoverished, fragile country. And it has little to do with the democratic aspirations of the youths who first took to the streets in January, inspired by the uprising in Tunisia.

There were worrying signs Thursday that the conflict was spreading and intensifying. Huge blasts shook the city late in the evening, and fighting forced the closure of Yemen’s main international airport, just outside Sanaa. The Defense Ministry said that special forces, which are commanded by Saleh’s son Ahmed, were attempting to retake government buildings in the capital, including ministries seized by tribal fighters.

The heightened violence has marginalized U.S. and regional efforts to arrange Saleh’s removal from power and orchestrate a peaceful transition to elections. Instead, after the Ahmars galvanized committed fighters over the past week or so, observers said the country was locked in a fight in which neither side has a decisive military edge.

Prolonged fighting would divert government forces from counterterrorism operations, create a vacuum that could be exploited by terrorists, and potentially affect the ability of the United States, which has deployed significant military and intelligence assets in Yemen, to strike at al-Qaeda in the Arabian Peninsula. Militants allied with the group have reportedly seized buildings in Zinjibar, the capital of the southern province of Abyan.

John O. Brennan, President Obama’s top counterterrorism official, was in Saudi Arabia on Thursday to discuss what the White House called “the deteriorating situation” in Yemen and was scheduled to continue those talks in the United Arab Emirates.

The other major armed player in Yemen, Gen. Ali Mohsen al-Ahmar, who is not directly related to the Ahmar brothers, has sat out the fighting. Mohsen, who commands Yemen’s 1st Armored Division, and his troops have protected protesters since several of them were killed by government snipers. But he has otherwise not deployed his forces against Saleh, who has ruled for 32 years.

Until relatively recently, the political and economic interests of the Saleh and Ahmar families seemed closely intertwined. But the demonstrations against Saleh’s autocratic rule have brought into the open long-simmering tensions about sharing the spoils of office, and, since May 23, forces loyal to Saleh have been engaged in fierce street battles with tribesmen loyal to the Ahmar family.

“This conflict between these groups is threatening to drag the country into a civil war, an outcome that would almost certainly overshadow the laudable aspirations of Yemeni protesters for meaningful regime change,” said April Alley, senior Arabian Peninsula analyst for the International Crisis Group.

Souring of relations

The Ahmar family had been allied with Saleh since it helped him ascend to the presidency of North Yemen in 1978. Abdullah al-Ahmar, the family patriarch, was Yemen’s preeminent sheik and the powerful head of the Hashid tribal confederation, which included Saleh’s family.

When Abdullah died in 2007, none of his 10 sons, including his designated successor, Sadiq, enjoyed the same standing, although they remained very wealthy insiders. As Saleh concentrated more and more power around himself, his son and his nephews, fissures began to develop between the two families, Alley said.

In the 2006 presidential elections, Hamid al-Ahmar, one of Yemen’s leading businessmen and Abdullah’s son, backed Saleh’s opponent. His father stuck with Saleh. By 2009, Hamid al-Ahmar was calling for Saleh’s ouster, saying in an interview on al-Jazeera, the pan-Arab television network, that the president was guilty of “high treason” and accusing him of turning Yemen into a family empire.

In March, after government troops fatally shot protesters, Sadiq al-Ahmar called on Saleh to leave office. He also paid several visits to the protest site in front of Sanaa University that became known as Change Square, to express solidarity with the protesters.

Saleh has thrice promised to resign and then balked, most recently late last month, when he rejected a deal that would have guaranteed him immunity from prosecution in exchange for transferring power. Instead, street fighting broke out, and Saleh’s forces fired on the Sanaa compound of Sadiq al-Ahmar, killing several prominent sheiks involved in a mediation effort and injuring dozens. None of the Ahmar brothers was killed.

“When the idea of a power succession started to emerge, the sons of al-Ahmar started to feel they were more eligible than Saleh’s son,” said Faris al-Saqqaf, director of the Center for Future Studies in Sanaa. “This created a tipping point in the relationship between the al-Ahmar family and Saleh, and that is why Saleh decided to attack them as his main political challenger.”

Analysts said that Mohsen, the general, may be attempting to position himself as a neutral patriot and that he may share some of the protesters’ suspicions about the Ahmars’ ambitions.

“The protesters are the ones talking about accountability and governance,” said Christopher Boucek, an associate in the Middle East program at the Carnegie Endowment for International Peace. “They don’t want to see one family replaced with another.”

Some protesters on Change Square said Saleh’s latest assault, particularly on the home of Sadiq al-Ahmar, has led some activists to reconsider their attitude toward the brothers.

“Many people, even tribesmen, used to have a negative impression about the al-Ahmar family,” said Eisa Abdullah, a protester in Sanaa. But with the attack on Sadiq al-Ahmar’s house, “Saleh has turned them into heroes.”

Brothers vs. government

Although Sadiq al-Ahmar is the ostensible head of the tribe, Hamid al-Ahmar is widely regarded as the one with political ambitions. Hamid, a tycoon with interests in telecommunications and construction who also holds the KFC and Baskin-Robbins franchises in Yemen, has sent mixed messages about whether he wants to succeed Saleh.

Hamid has said that someone from southern Yemen should probably be the next president to heal some of the deep regional divisions in the country. But in a 2008 interview with Victoria Clark, the author of “Yemen: Dancing on the Head of Snakes,” he said he was willing to challenge Saleh and become president.

“I’m proud and willing to do the job,” he said. “When you want to help your country, you don’t count the danger.”

The Ahmars and their allies have stockpiled weapons, including heavy artillery, mortars and rocket-propelled grenades. Military officials loyal to the president warned that fighter jets would be used to attack the tribesmen if they advanced toward the capital. As if to stress that point, jets flew low over the capital and outlying areas Thursday.

“The brothers don’t have the capacity to overthrow the government, and the government doesn’t have the capacity to eliminate the al-Ahmar brothers,” Boucek said. “This can go on for a long time, and it could get a lot worse.”

A special correspondent in Sanaa contributed to this report.