Yemen’s political crisis spiraled further toward chaos Monday, as five key generals defected to join anti-government protesters, further weakening longtime President Ali Abdullah Saleh’s tenuous hold on power.

The generals, who lead much of the nation’s armed forces, broke from the president after government loyalists killed more than 50 protesters Friday during demonstrations in the capital, Sana.

Talk of a coup swirled in the strategically situated nation, with tanks rattling through the streets of the capital, Sana, as soldiers loyal to one of the defecting commanders joined protesters while those siding with Saleh took positions around the presidential palace.

After more than decades of manipulating tribes and political opponents to remain in power, Saleh has seen the clamor for his ouster spread from the streets to the ruling elite, including a respected tribal leader, who in recent days has stood with protesters.

The protests have shifted from a carnival-like enthusiasm — with tribesmen with traditional daggers around their waists dancing alongside students in T-shirts — to battlefield somberness.

“”Saleh is clearly on his way out. There is no turning back,”” said Barbara K. Bodine, U.S. ambassador to Yemen from 1997 to 2001 and a diplomat in residence at Princeton University. “”His government has left him. The defections have now become a flood.””

Yemen is at the strategic crossroads of the Middle East and the Horn of Africa. U.S. and Western officials have long feared that a political meltdown with chronic unemployment, malnutrition, and drought would ignite further instability. Saleh, who once described Yemen as dancing on the heads of snakes, no longer has the tight grip on the country he once had.

The mercurial ruler’s predicament is one in the series of revolts that have gripped the Middle East, toppling the presidents of Egypt and Tunisia. It is uncertain what chaos would unfold if Saleh were no longer ruling a nation already is facing a rebellion in the north, a secession movement in the south and the highly active al-Qaida in the Arabian Peninsula.

Although regarded as brutal and corrupt, Saleh is a U.S. ally against the Islamic militant network that in the past two years has staged bloody attacks across Yemen and has taken credit for unsuccessfully attempts to blow up American airplanes. His departure would also trouble neighbor Saudi Arabia, which sent troops into Yemen in 2009 to contain Houthi rebels in the north and to seal its border from Al Qaeda operatives.

“”I think we have to be very concerned”” about Yemen, said Rep. C.A. “”Dutch”” Ruppersberger of Maryland, the ranking Democrat on the House Intelligence Committee. “”It’s a problem even as we speak, whether he falls or not, because it’s a very weak government. What concerns me is it’s a training ground for al-Qaida..””

In recent weeks, an awkward alliance of opponents — Islamists, reform-minded college students, socialists and tribal sheiks — have participated in a series of massive protests in Sana, their common goal to dislodge Saleh but their visions for the future diametrically different.

The five generals who defected Monday include Maj. Gen. Ali Mohsen al-Ahmar, a distant relative of Saleh’s and commander of the 1st Brigade in the country’s northwest.

“”The state, represented by the president, is totally responsible for the blood that was shed,”” al-Ahmar told Al Jazeera news channel, noting that his defection was “”an answer to the developments in the streets.””

Al-Ahmar has been linked to corruption and is said to have sympathies for radical Wahhabi conservatives, according to U.S. diplomatic cables recently released by WikiLeaks. He has been viewed as close to Saleh’s inner circle and is not likely to be embraced by young dissidents seeking democratic reforms.

There were indications late Monday that Saudi Arabian officials were attempting to mediate a possible graceful exit for Saleh. But the president was characteristically defiant as protests continued. He was quoted by the state news agency as saying: “”We’re still here. . . . Those who are calling for chaos, violence, hate and sabotage are only a tiny minority.””

Protests continued near Sana University, joined by troops from al-Ahmar’s 1st Brigade. “”This is our most important victory so far,”” said Issam Badr, a 19-year-old pharmacist who has attended the demonstrations since they began early last month.

“”No more thugs can kill us with impunity,”” he said. “”Everybody here welcomes the 1st Brigade and salutes its soldiers.””

The government will not “”allow under any circumstances an attempt at a coup against democracy and constitutional legitimacy,”” Brig. Gen. Mohammad Nasser Ali, the defense minister, said on national television. “”The armed forces will stay faithful to the oath they gave before God, the nation and political leadership under the brother President Ali Abdullah Saleh.””

But the president’s uncanny ability to outflank his enemies is under a new level of strain from more than the increasingly divided military. Sheik Sadiq Ahmar, a key tribal leader who last month resigned from the ruling party, has also joined protesters. The sheik is head of a tribal federation that includes Saleh’s clan and has been competing with the president for tribal loyalties in Yemen’s north.

On Sunday, after several officials quit, Saleh fired his entire Cabinet in his latest attempt to appease demonstrators. He has also met with opposition parties but, so far, has not been able to convince his detractors that he should remain in office until presidential elections in 2013. In an effort to calm protests, Saleh promised last month that he would not seek re-election.

At least eight ambassadors and a number of legislators from the ruling General People’s Congress have resigned. Saleh, though, has for years sought to consolidate power in his family; his son, half-brother and nephews hold key military and intelligence posts. It is uncertain who would rise in a political vacuum, but they would need the endorsement of key tribes.

“”Saleh is more and more isolated, and less and less responsive to advice from those practical, progressive”” government insiders, according to a 2005 U.S. diplomatic cable released by WikiLeaks. The documents go on to quote embassy contacts in the Yemen government as saying that “”Saleh “”listens to no one,”” and is “”unrealistically and stupidly confident”” that he will always make the right decisions.””

“”Saleh . . . does not think strategically and cares only about enriching his own family.””

Over the years, Gen. al- Ahmar had been close to Saleh. A 2005 U.S. diplomatic cable suggested that al-Ahmar was one of the elite who benefitted from Saleh’s corrupt regime by smuggling arms and diesel. He is also, according to former U.S. officials, more inclined toward radical political Islam than is the president.

His “”questionable dealings with terrorists and extremists . . . would make his accession unwelcome to the U.S. and others in the international community,”” said a cable, signed byThomas Krajeski, who was then the U.S. ambassador in Sana.

“”He is known to . . . support a more radical Islamic political agenda than Saleh. He has powerful Wahhabi supporters in Saudi Arabia and has reportedly aided the 1/8Saudis3/8 in establishing Wahhabi institutions in northern Yemen.””