The fire at a 12-unit apartment building in the 200 block of West California Avenue was reported shortly after 3 p.m., Capt. Dave Pitts said.
Around 4 p.m. the fire had been controlled, Pitts said.
He said many units sustained severe damage in the blaze and residents will not be able to return to their homes.
The Sunnyvale Town Center Apartments are listed in that block, which was shut down between North Murphy and North Sunnyvale avenues.