The fire was reported at about 3:40 a.m. at a home in the 2100 block of Russell Street, a dispatcher said.
Berkeley Deputy Fire Chief Gil Dong said a neighbor reported hearing someone yelling for help.
Firefighters arrived to find a two-and-a-half-story Victorian house ablaze, with flames visible in the rear of the home, Dong said.
An elderly couple who live in the house evacuated safely on their own, he said.
The fire was not easy to access, Dong said.
"Crews had difficulty making entry because the occupants had a lot of personal possessions that were obstructing the doorways and the windows, he said.
About two dozen firefighters battled the fire, bringing it under control at about 5:30 a.m., Dong said.
No injuries were reported, and the damage from the fire has not yet been assessed.
Crews remained at the home mid-morning participating in an "extensive overhaul process due to the number of possessions in the home," Dong said.
The cause of the fire remains under investigation.