Witnesses say rebel Houthi militiamen are battling soldiers near Yemen's presidential palace.
The status of President Abed Rabbo Mansour Hadi was not immediately clear, AP reported.
The battle began Monday morning. Witnesses say they heard heavy machine gun fire and mortars falling in the neighborhood. Civilians in the area fled the fighting.
There was no word on state media about the violence as Sanaa was suffering a power outage at the time.
The Houthis seized large areas of Yemen, including Sanaa, last year as part of their protracted power struggle with Hadi. Critics say the Houthis are a proxy for Shiite Iran, charges the rebels deny.