SANA, Yemen — The tenuous power of Yemen’s embattled president weakened Monday as a wave of high-level officials, including the country’s senior military commander, an important tribal leader and a half-dozen ambassadors, abandoned him and threw their support behind protesters calling for his ouster.

As the country girded for the next stage of a deepening crisis, military units appeared to take sides in the capital, with the Republican Guard protecting the palace of President Ali Abdullah Saleh and soldiers from the First Armored Division under the defecting military commander, Brig. Gen. Ali Mohsin al-Ahmar, protecting the throngs of protesters in Sana.

Despite a celebratory mood among the demonstrators, the standoff prompted the United States Embassy to urge Americans in Yemen to stay indoors on Monday night because of “political instability and uncertainty.”

The defection of General Ahmar, who commands forces in the country’s northwest, was seen by many in Yemen as a turning point, and a possible sign that government leaders could be negotiating an exit for the president. But the defense minister, Brig. Gen. Muhammad Nasir Ahmad Ali, later said on television that the armed forces remained loyal to Mr. Saleh.