The explosion and fire were reported just before 1:30 a.m. in a multi-unit apartment building at 1304 Mariposa St., the dispatcher said.
Three people suffered minor injuries, according to dispatch. It was not immediately known if the victims were treated at the scene or hospitalized.
Units from Contra Costa County fire responded to a two-alarm blaze that erupted after the explosion, the dispatcher said. The fire was under control by 2:10 a.m.
As of 4:30 a.m., the cause of the explosion was still under investigation, according to the fire district.