Explosions were heard throughout Sanaa from early hours and massive plumes of smoke and fire seen rising from several neighbourhoods where opposition forces are stationed, Witnesses and AFP correspondents in Sanaa said. There was no immediate word on casualties.
An AFP correspondent said ambulances were seen racing out of the Hasaba district that is home to tribal chief sheikh Sadeq al-Ahmar and his brothers, whose armed tribesmen have been battling Saleh's troops for weeks.
In the Change Square area, where thousands of protesters have been camped out calling for Saleh to quit, fierce clashes erupted between government troops and defected soldiers loyal to dissident General Ali Mohsen al-Ahmar.
Mohsen and Sadeq, former staunch allies of Saleh, are now his arch-rivals, each having thrown their support behind the nationwide protests which have triggered the political crisis sweeping Yemen.
Saturday's violence came a day after the UN Security council unanimously passed a resolution that "strongly" condemns the deadly government attacks on demonstrators and backs a Gulf plan for Saleh to end his 33 years in power.
Several hundred people have been killed since the protests against Saleh erupted in late January.
Saleh has for months refused to buckle under intense local, regional, and international pressure to resign and hand power over to his deputy, Vice President Abdrabbo Mansour Hadi.
Under a Gulf Cooperation Council (GCC) initiative, Saleh must step down within 30 days of signing the deal handing him and his family immunity from prosecution, a controversial component of the transition plan.
Yemen's Nobel Peace Prize winner Tawakkul Karman criticised the United Nations resolution on Friday for supporting the immunity clause in the plan drawn up by Yemen's energy-rich Arab neighbours in the Gulf.
"They have to discuss the ousting of Saleh and how he has to be handed over to the International Criminal Court as a war criminal," Karman said outside the UN in New York where she has been calling for a tougher UN stance on Saleh.
Calls for more decisive UN action have increased, with tens of thousands of pro-democracy activists urging the world body to step in and stop the violence engulfing the impoverished country.
The protesters have also increased their calls for Saleh to be tried before the world court.
Britain's UN ambassador Mark Lyall Grant said the resolution does include some "tough messages" for Saleh and that there would be "no impunity for human rights abuses," despite its clear support for the GCC initiative.