SANAA, Yemen – Rival tanks deployed in the streets of Yemen's capital Monday after three senior army commanders defected to a movement calling for the ouster of the U.S.-backed president, leaving him with virtually no support among the country's most powerful institutions.

Maj. Gen. Ali Mohsen al-Ahmar, commander of the army's powerful 1st Armored Division, was the most senior of the three commanders to join the opposition. He announced his defection in a message delivered by a close aide to protest leaders at the Sanaa square that has become the epicenter of their movement.

Some of the tanks and armored vehicles deployed in the Sanaa square where protesters have been camping out to call for the resignation of President Ali Abdullah Saleh, whose forces opened fire from rooftops and killed more 40 demonstrators on Friday. Others were deployed at state TV, the Central Bank and the Defense Ministry.

Saleh, who has cooperated closely with a U.S.-backed offensive against his nation's branch of al-Qaida, looked to be far closer to what analysts increasingly have called inevitable: a choice between stepping down after 32 years in power or waging a dramatically more violent campaign against his opponents.

A senior opposition leader said contacts were underway with the president over a peaceful way out of the ongoing crisis. One option under discussion, he said, was for Saleh to step down and a military council takes over from him to run the country till presidential and legislative elections are held.

The leader, who spoke on condition of anonymity because of the sensitivity of the contacts, declined to say how much progress the talks have made, but gave 48 hours as the likely timeframe for a breakthrough.

Also Monday, Saleh sent his foreign minister to Saudi Arabia, Yemen's powerful neighbor and the on-and-off backer of the Yemeni leader, with a message to King Abdullah. The contents of the message were not known.

At least a dozen tanks and armored personnel carriers belonging to the Republican Guards, an elite force led by Saleh's son and one-time heir apparent, Ahmed, were deployed outside the presidential palace on Sanaa's southern outskirts, according to witnesses.

The deployment appeared designed to counter the presence of elements of the 1st Armored Division elsewhere in the city.

All three officers who defected belong to Saleh's Hashid tribe. A Hashid leader said the tribe, eager to keep the president's job for one of its own, was rallying behind Maj. Gen. al-Ahmar as a possible replacement for Saleh.