Hundreds of tourists have been transported to safe places by helicopter after flash floods submerged the camps in southern France.
A 70-year-old German man who was helping supervise children in a camp was reported missing on Thursday after he was dragged into his caravan by the flood waters, police said.
The vehicle was found "empty and in pieces" near the river after the man was caught by a muddy and fast-flowing torrent when the Ardeche River overflowed.
About 1,600 people were evacuated from several camps after torrential rains increased the rivers in the Gard region, and four German children were also taken to the hospital with hypothermia.
Rescue teams go through damaged bicycles in a flooded camp in southern France when storms and heavy rains hit the country
A car is swallowed by water in Bourg as flash floods hit southern France after the recent heat wave gave way to a storm.
The divers were taking part in the search for the missing man, said an AFP journalist at the scene.
The mayor of Bagnols, Jean-Yves Chapelet, told BFM-TV that many Dutch tourists were among the 350 campers evacuated there. A group of 136 children and young people from the German city of Leverkusen were reported as safe.
"The situation is extremely dangerous," said a spokesman for the emergency services in the city of Bagnols-sur-Ceze, north of Avignon, where the drama occurred.
"A German retiree is still missing after being dragged in. Every effort is being made to try to find him, with water channels being searched where possible." Five camps in the area have been evacuated.
The Gard is very popular with British tourists in August, while many others from the United Kingdom have retreated to the area or have second residences there.
The spokesman said: "Like everyone else, they should remain on guard, our advice is to find high ground and avoid unnecessary travel."
He added: "Helicopters are being used in relief operations, with valleys particularly badly hit."
Videos posted on social networks showed that flood water was rushing through populated areas.
& # 39; Nobody has suitcases. We just have what we're wearing, "France Info Rita Mauersberger, a visitor from Germany who was among the campers sheltering in a local lounge in Saint-Julien-de-Peyrolas, told France Info radio.
A campground flooded in Saint-Julien-de-Peyrolas, in the south of France, where the warm weather has given way to storms and floods
A rescuer stands next to a damaged tent while emergency services respond to floods in southern France
More than 400 firefighters and gendarmes, many sent from other regions, helped in the evacuations, used helicopters to detect the camps and make rescues.
Numerous roads in the area remained cut off at nightfall. The authorities warned that the flood would take time to retreat and urged people to be vigilant.
The European heat wave sent the mercury above 113 ° F (45 ° C) last weekend, intensifying the forest fires that began last Friday. A sparkling heat wave throughout Spain has left 10 people dead in a week.
A spokesman for the Prefectures of Gard said: "The floods are expected to peak at around 11 o'clock on Thursday night."
Authorities also said that 119 children in nearby Saint-Julien-de-Peyrolas were sent to an emergency shelter as hundreds of firefighters supported by four helicopters assisted in the evacuation.
The images show rescuers wade through flooded camps with damage to tents, caravans and bicycles.
Several parts of central and southeastern France are under flood warning after weeks of high temperatures that erupted in storms on Thursday, turning several camps into mud baths. Around 17,000 houses in the southwest and northeast were without electricity.
Electricity supplies have been cut off in some villages, while roads have stalled due to the number of people trying to reach high ground.
Storms have also hit in Germany where there is lightning behind the Reichstag building in Berlin on Thursday night
Thunderbolts are seen over the United States embassy near the Brandenburg Gate in Berlin when storms hit Germany on Thursday
Dark clouds hang over a field with windmills while lightning flashes across the sky over Sehnde near Hannover, Germany
A rescuer walks in front of a damaged caravan in a camp in Saint-Julien-de-Peyrolas, in the south of France
Floods have spread through southern France as the European heatwave gives way to dramatic storms
Hundreds of campers need to rescue, while others have faced power cuts in the midst of flash floods in southern France