Tanks were deployed in the Yemeni capital on Monday as top Generals pledged allegiance to the “revolution” and the country's main tribal leader demanded President Ali Abdullah Saleh's exit.

But even as some of his closest allies abandoned him, the embattled leader refused to submit to calls for his resignation and claimed he had the support of the vast majority of people in the impoverished country.

Tanks took up positions in key locations across Sana'a including at the presidential palace, the central bank and the Ministry of Defence, but it was unclear what their orders were or who was in command.

In the first of a series of body blows to Mr. Saleh's authority, General Ali Mohsen al-Ahmar, commander of the Northwest Military District which includes Sana'a, announced he had joined the “revolution”. “The crisis is getting more complicated and it's pushing the country towards violence and civil war,” the General said.

One by one, dozens of officers of various ranks stood at the tent city near Sana'a University, where demonstrators have kept vigil since February 21 in spite of a wave of attacks, and publicly pledged to support the revolution.

Sadiq al-Ahmar, who leads the Hashid tribal federation, the largest in deeply tribal Yemen and a crucial source of Mr. Saleh's power, told Al-Jazeera it was time for the embattled President to make a “quiet exit”. “I announce in the name of all the members of my tribe that I am joining the revolution,” Mr. Ahmar said,

Deputy Speaker of Parliament Himyar al-Ahmar and Governor of the key southern province of Aden Ahmed Qaatabi also resigned in protest at the treatment of demonstrators.

The defections came a day after Mr. Saleh sacked his Cabinet in a bid to placate opposition calls for sweeping reforms in the key U.S. ally.

As the pillars of his power apparently collapsed beneath him, Mr. Saleh sent Foreign Minister Abu Bakr al-Kurbi on an urgent mission to Saudi Arabia with a secret message for the King, the state news agency reported.

The regime has already lost the support of religious leaders and been weakened by the resignations of Ministers, Ambassadors and a host of ruling party MPs, but Mr. Saleh has refused to stand down until his term ends in 2013.

His regime was internationally condemned after more than 50 people were killed when loyalist gunmen opened fire Friday on protesters in Sana'a's University Square, the centre of the pro-democracy movement.

The defection of top military officers to the opposition is likely to complicate Washington's support for Mr. Saleh, whom it sees as a guarantor of stability in a volatile country and a partner in the war against Al-Qaeda.

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