Forest fires raged through parts of France's Mediterranean coast on Thursday, with the worst in almost two decades burning around Marseille, injuring seven people and forcing authorities to evacuate thousands from their homes.

About 2,500 firefighters still battled the blazes that broke out on Wednesday, backed up by firefighting airplanes from France and neighbouring Italy, Interior Minister Bernard Cazeneuve told journalists.

A 50-year old man taking pictures of the fires in Vitrolles, 20 km northwest of Marseille, was being questioned by police, a source close to the investigation told Reuters, but the interior minister said it was too early to say if criminal intent was behind the fires.

The fires, fanned by strong northern winds known as the mistral, ravaged more than 2,000 hectares of the dry, pine-planted hills north of Marseille and were stopped just at the outskirts of France's second-biggest city.

"The drought of the last few months and the strong mistral winds worked like a powder keg," Richard Mallie, head of the firefighting service said on France Bleu Provence radio. "The fires spread at phenomenal speed, 2,500 metres an hour."

The situation was under control on Thursday, but fires were still active, local authorities said. The areas concerned are densely populated but are not those most popular with tourists, located mostly further east of Marseille.

More than 20 houses burnt down around Vitrolles. A local newspaper, La Provence, showed pictures of a secondary school in the town of Les Pennes-Mirabeau destroyed by the flames.

The fires came close to the petrochemical complex of Fos-sur-Mer, which includes several refineries and oil depots. Only slight damage had been reported at the minerals terminal and none at the oil terminal, however.

After some delayed and canceled flights late on Wednesday, the situation at Marseille airport was returning to normal, it said on its website, although traffic was disrupted because some roads leading to the airports were still closed.