Tanks have been deployed in the Yemeni capital as top generals pledge allegiance to the "revolution" and the country's main tribal leader demand President Ali Abdullah Saleh's exit from power.
Tanks took up positions in key locations across Sanaa, 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 a series of body blows to Saleh's authority, General Ali Mohsen al-Ahmar, an armoured infantry division commander, announced he had joined the "revolution", along with tribal leader Sadiq al-Ahmar.
"The crisis is getting more complicated and it's pushing the country towards violence and civil war," the general said in a statement broadcast by al-Jazeera television.
"According to what I'm feeling, and according to the feelings of my partner commanders and soldiers... I announce our support and our peaceful backing to the youth revolution.
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"We are going to fulfil our duties in preserving security and stability."
One by one, dozens of officers of various ranks stood at the tent city near Sanaa University, where demonstrators have kept vigil since February 21 in spite of a wave of attacks, and publicly pledged to support the revolution.
Another top general, Nasser Ali Shuaybi, said he had also defected along with 60 military officers and 50 police personnel in Hadramawt province.
Sadiq al-Ahmar, who leads the Hashid tribal federation, the largest in deeply tribal Yemen and a crucial source of Saleh's power, told al-Jazeera television 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," Ahmar said, calling for Saleh "to exempt Yemen from the bloodshed and make a quiet exit".
The governor of the key southern province of Aden, Ahmed Qaatabi, also resigned "to protest what is happening in the country", an aide said.
The defections came a day after Saleh sacked his cabinet in a bid to placate opposition calls for sweeping reforms in the key US ally.
The regime has lost the support of religious leaders and been weakened by the resignations of ministers, ambassadors and a host of ruling party MPs, but Saleh has refused to stand down until his term ends in 2013.
His regime was internationally condemned after more than 50 people were killed as loyalist gunmen opened fire on protesters in Sanaa's University Square, the centre of the pro-democracy movement.
