The island of Corsica, the town of Carros, close to Nice, and Saint-Tropez were among the areas badly hit on Monday.
Hundreds of homes were evacuated as a precaution.
"It's a very dangerous day," Michel Bernier, of France's civil defence forces, told AFP news agency.
"And the fight is going to be very, very long tonight," he added as firefighters worked into Tuesday.
Unusually hot, dry and windy weather in south-east France has contributed to the spread of forest fires across the region.