Senior figures in the ruling party of Yemen have formed an opposition bloc, nominally in support of the mass anti-government demonstrations that have gripped the country. Top military officers have also moved against President Ali Abdullah Saleh.

Several leaders of the General People’s Congress, the party of President Saleh, formed the Change and Construction bloc on Tuesday. Saleh’s son-in-law and head of the Yemen Oil Company Omar al-Arhabi resigned, announcing his support for the demonstrators. The Hayel Saeed Group, the business empire of one of the country’s wealthiest families, joined him in backing what they called the “revolution.”

Among other senior figures, Himyar al-Ahmar, deputy speaker of the parliament, and Ahmed Qaatabi, the governor of the city of Aden, resigned from the party and declared their support for the protesters. Several Yemeni ambassadors also quit their posts.

Resigning his post, Yemen’s ambassador to Saudi Arabia, Mohammed al-Ahwal, told the AFP news agency that he had given his support to “the youth revolution, and for change in Yemen.”

Tribal leaders have also moved to oppose Saleh. Sheikh Sadiq al-Ahmar, the leader of Saleh’s own Hashed tribe, issued a call for the president to step down. The statement was backed by several leading Muslim clerics.

The move within the ruling party came after several of Yemen’s top military commanders announced that they had split with Saleh, and that the forces under their command would not adhere to the president’s orders.

Major General Ali Mohsen, who controls Yemen’s northwest military zone, told Al Jazeera on Monday that he was deploying his forces to Sanaa to “protect the youth revolution.”

“Lack of dialogue and oppression of peaceful protesters in the public sphere resulted in a crisis which has increased each day,” the general told a news conference on Monday. He added, ominously, that his armored division located near to Sanaa would “fulfill our complete duty in keeping the security and stability in the capital.”

Sometimes called Ali Mohsen Saleh because of his family connections to the president and other times known as Ali Mohsen al-Ahmar, he is one of Yemen’s richest and most powerful figures. The general had been a close ally of Saleh in the 1990s, but fell afoul of the president in 2009 and has since been marginalized within the regime.

Ali Mohsen clearly views the current crisis and power vacuum in Yemen as an opportunity to advance and secure his power and wealth at the expense of Saleh, and perhaps even take over the presidency.

He appears to head a large group of senior military commanders who defected soon after his announcement. Brigadier General Mohammad Ali Mohsen, head of military forces in the east of the country, as well as commanders in Hadramawt and Lahij provinces, and other top brass also mutinied against Saleh on Monday.

“It is officially over, now that 60 percent of the army is allied with the protesters,” Hakim al-Masmari, editor-in-chief of the Yemen Post, told Al Jazeera on Monday evening. “For Ali Mohsen Saleh to announce this, it is a clear sign to President Saleh that the game is over and that he must stop now.”

Al-Masmari went on to state that he expected “90 percent” of the armed forces to move against the regime by Tuesday. However, al-Masmari added that Maj. Gen. Ali Mohsen would “not be accepted by the youth,” who wanted a civilian government.

President Saleh’s defense minister, Mohammad Nasser Ali, responded to the military split, announcing on national television that, “The armed forces will stay faithful to the oath they gave before God, the nation and political leadership.”

“We will not allow under any circumstances an attempt at a coup against democracy and constitutional legitimacy, or violation of the security of the nation and its citizens,” the defense minister added.

As of this writing, the National Defense Council, the supreme military body in the country, still backs Saleh. Tanks and troops loyal to Saleh are located near key government buildings and the presidential palace in the capital.

In a desperate attempt to stave off a coup, Saleh sent his foreign minister, Abu Bakr al-Kurbi, to Riyadh to appeal to the Saudi monarchy to intervene. Saudi Arabia has already sent its troops to the Persian Gulf island kingdom of Bahrain in order to crack down on protests there.

The Saudis, working with Washington and the Saleh regime, have in recent years launched military attacks on tribes inside Yemen. The Saudi monarchy is concerned about the spread of tribal rebellion into Saudi Arabia, and will be wary that the ouster of Saleh could leave a power vacuum on its southern border.

The defection of a significant section of Saleh’s top backers is an expression of the precarious status of the regime. Weeks of repression against mass protests of workers and youth in Sanaa and other cities have failed to quell the protests. The massacre of over 50 demonstrators near the university in the capital last week, with security forces firing on unarmed protesters from the roofs of buildings, was the latest and bloodiest attempt by the regime to cling to power.

The attack backfired, however. Hundreds of thousands of demonstrators flooded onto the streets of Sanaa over the following days, joined by thousands of tribesmen who came into the city promising to defend protesters from any further attacks.

The recent intensification of various tribal conflicts with the government in Sanaa, which have been going on for years, has rendered Saleh virtually powerless across much of the country. The United States, under the guise of the “war on terror,” has been assisting Saleh’s conflict with some tribes, with US drones allegedly being used to assassinate tribal leaders. US special forces have reportedly been training their Yemeni counterparts in covert surveillance and assassination techniques.

There is a powerful secessionist movement in the south of Yemen. North and South Yemen had been separate countries until 1990, with Saleh ruling the northern US-backed state since 1978, while the south was within the Soviet sphere of influence during the Cold War. Since protests broke out in late-January, the regime has been able to exert little control in the major southern city of Aden, where protesters have attacked Saleh’s officials and taken over police stations and government buildings.

The alliance of powerful tribal groups with the movement of workers and youth in the cities poses the threat of full-scale civil war in Yemen. It is in order to bring the protests and tribal insurrections under control, and therefore to secure the rule of the Yemeni bourgeoisie, that the generals and ruling party figures have adopted an oppositional stance.

The military top brass and all the political leaders now deserting Saleh’s government have profited enormously from the exploitation of the Yemeni people, the poorest in the Arab world. There are reports of some generals controlling millions of dollars in trade from smuggling and military-related business.

The military’s subservience to US imperialism has also been lucrative. The Saleh regime and the military saw aid from Washington double last year, as the Obama administration stepped up US intervention in the country.

Saleh’s principal remaining constituency of support is to be found not in Yemen, but in the White House, the Pentagon and the State Department. The Obama administration has backed Saleh throughout the protests, only offering the most muted criticisms of his murderous campaign against his own people. The comment from the US State Department, issued after 52 protesters were gunned down in the streets of Sanaa, merely called on the Yemeni government to “investigate these incidents.” The statement went on to call on “all parties to engage in a process of peaceful negotiation”—suggesting that the demonstrators were as much to blame for being shot as the police were for shooting them!

While raining bombs and missiles on Libya on the pretext of the Gaddafi regime’s killing of civilians, the US government has stood back and allowed its long-time ally in Sanaa to put down the protests by force. With the fall of Saleh seemingly imminent, US imperialism will seek to maintain its interests in this strategically vital country—near to the main oil fields and shipping lanes of the Persian Gulf—through the connections it has cultivated for years with the military and other former Saleh loyalists.