French authorities said swathes of the Var area of Provence had been evacuated, involving the displacement of some 10,000 people - including 3,000 campers on Tuesday night.
Local emergency services said flames had engulfed some 4,000 hectares (9,880 acres) of land in southern France, where the population swells during summer because of popular tourist resorts.
This included a separate fire in northern Corsica.
Tourists were seen camped out on beaches overnight - while the glare of the morning sun was darkened by rising smoke, images posted to social media showed.
Sheffield couple David Heslop, 81, and his wife Carolyn, 78, were forced to camp on a beach with their friends Anne Davies, 74 and her husband John, 74.
Speaking to the Daily Mail, Mr Heslop said: "My biggest fear was the wind was going to change. The fire would have engulfed us. There must be 500 gas canisters and 500 petrol tanks in the campsite. It would have been carnage."
Mrs Davies said "We've been coming here for 34 years and I've never seen fires this big. When I looked back the whole of the hillside was on fire.
"I was very frightened, particularly as I can't run that fast any more. The beach was full of people fleeing from the fire and there must have been about 50 children in buggies with worried parents."
Hundreds of firefighters are battling to bring several blazes under control since they broke out on Monday, including the one which has scorched 800 hectares in Var.
Four tracker planes and a firefighting aircraft had formed part of the emergency response after the intensity of the fire grew at around 11pm on Tuesday, a statement from the Var prefecture said.
French authorities were forced to turn to the European Union to request help as the fires continued.