Three senior army commanders defected to anti-government ranks in Yemen today and ordered their tanks on to the capital's streets in a show of force.
Maj. Gen. Ali Mohsen al-Ahmar, commander of the army's powerful 1st Armoured Division, announced his defection in a message delivered by a close aide to protest leaders at the square in Sanaa that has become the centre of their movement.
Some of the division's tanks and armoured vehicles then deployed in the square, which protesters have occupied for more than a month to call for the resignation of President Ali Abdullah Saleh after 32 years in power.
An increasingly violent crackdown on the demonstrations escalated dramatically on Friday when Mr Saleh's forces opened fire from rooftops, killing more 40 in an assault that caused much of his remaining power base to splinter.
Maj. Gen. al-Ahmar also sent tanks to the state television building, the Central Bank and the Defence Ministry.
Mr Saleh, who has co-operated closely with a US-backed offensive against his nation's branch of al-Qa'ida, looked to be far closer to what analysts increasingly have called inevitable: a choice between stepping down or waging a dramatically more violent campaign against his opponents.
He appeared to be retaining the loyalty of at least some of Yemen's military.
Defence Minister Mohammed Nasser Ahmed said that the armed forces remained loyal to the president and would counter any plots against "constitutional legitimacy" and "democracy."
He spoke after a meeting of the National Defence Council, which is led by Mr Saleh and includes the prime minister, the defence and information ministers as well as the intelligence chief.
At least a dozen tanks and armoured personnel carriers belonging to the Republican Guards, an elite force led by Mr Saleh's son and one-time heir apparent, Ahmed, were deployed outside the presidential palace on Sanaa's southern outskirts.
A senior opposition leader said contacts were underway with the president over a peaceful way out of the crisis. One option was for Mr Saleh to step down and a military council take over from him to run the country until presidential and legislative elections are held.
All three army officers who defected belong to Mr Saleh's Hashid tribe and a tribal leader said it was rallying behind Maj. Gen. al-Ahmar as a possible replacement, eager to keep the president's job for one of its own.
Mr Saleh fired his entire Cabinet yesterday ahead of what one government official said was a planned mass resignation, a series of ambassadors have quit in protest and Sadeq al-Ahmar, the chief of the Hashid tribe, said Mr he too was joining the opposition.
Maj. Gen. al-Ahmar has been close to Saleh for most of the Yemeni president's years in power. He has close associations with Islamist groups in Yemen that are likely to raise suspicions in the West about his willingness to effectively fight al-Qa'ida.
He is a veteran of the 1994 civil war that saw Mr Saleh's army suppress an attempt by southern Yemen to secede.
His defection to the opposition was welcomed by protesters, but the warm reception may not guarantee him a political career in a post-Saleh Yemen given his close links to the president.
Popular among troops and viewed as a seasoned field commander, Gen. al-Ahmar also has widely been seen as a rival to the president and his son, who saw him as a threat to him succeeding his father.
The two other officers who announced their defection were Mohammed Ali Mohsen and Hameed al-Qusaibi, who both have the rank of brigadier. Yemen's ambassadors to Jordan, Syria and parliament's deputy speaker also announced they were supporting the opposition, further undermining Mr Saleh's weakening authority.