In most of the blazes, parked cars were torched and the flames spread to nearby homes or apartment buildings.
Los Angeles police were put on tactical alert at 5 a.m. to help secure crimes scenes, protect evidence and boost patrols.
Police say at least 15 vehicles have been set ablaze. In some locations, more than one car burned.
A city spokesman says one firefighter was treated and released after being injured.
No one has been arrested for Friday's fires. But the spokesman says a man picked up Thursday on allegations he set three fires in the same area is still in custody.