The fire was reported at 4:45 a.m. at 175 S. Capitol Ave., and was upgraded to two alarms when firefighters found flames in the building's shared attic space, Capt. Mike Van Elgort said.
Crews controlled the fire by 6:15 a.m., he said.
Southbound lanes on Capitol Avenue were blocked between Florence and Alum Rock avenues while the fire was burning, Van Elgort said. The lanes reopened at 7:48 a.m., he said.
Van Elgort said nobody was in the building at the time the fire started, and that there were no injuries.
Three businesses sustained minor damage, mainly from water, Van Elgort said.
The fire is not considered suspicious but the cause remains under investigation, he said.