Top generals, ambassadors and some tribes in Yemen have thrown their support behind anti-government protesters.
The defections are seen as a major blow to President Ali Abdullah Saleh's efforts to ride out demands for his immediate exit.
A string of officials have broken ranks in recent days although some key military allies remain.
Defence Minister Mohammad Nasser Ali says the army still backs the president, setting the stage for a possible standoff with those commanders who threw in their lot with tens of thousands of protesters who have taken to the street for weeks.
France has become the first Western power to call publicly for Mr Saleh to stand down.
The president has been in power for 32 years throughout a civil war, numerous uprisings and militant campaigns.
Tanks were deployed outside the presidential palace in the southern port city of Aden as part of enhanced security.