Police in the Lozère region blocked off an area in the Cevennes national park where up to 10,000 people, including small children, attended the illegal rave at the weekend, local authorities said.
After initially being strictly locked down by the security forces, the partygoers were being gradually allowed to leave and by Monday afternoon around 4,500 people were still present at the site, local Préfet Valerie Hatsch, said in a statement.
Police had been guarding the secluded site in a protected area of the national park, preventing vehicles from leaving “for health reasons linked to the risk of spreading Covid-19,” the local authorities earlier told AFP.
They initially blocked off the area at the weekend to prevent new partygoers joining. Police were now checking the alcohol levels of those leaving and ensuring there were no traffic blockages on roads at the height of the season.
Many people were still so inebriated that they were unable to drive anyway from the site on the Causse Mejean plateau, officials said.
Some 10,000 people in hundreds of cars had streamed to the site around midnight on Saturday for a rave party that flouted a ban on gatherings of more than 5,000 people imposed due to the coronavirus outbreak.
Lozère, France's least populated department, has been spared the worst of the pandemic, which has so far claimed more than 30,000 lives nationwide.
Locals were furious about the risks posed by the event. Authorities said the revellers did not wear masks and ignored social distancing rules.
“People from Lozère take Covid very seriously, they have respected the distancing measures and this massive influx of people who flout all the rules has shocked them profoundly,” Hatsch said on Sunday.
Farmers were also angry that the partygoers invaded pasture and fields, damaging them in a national park where farmers themselves operate only under restrictions.
Police handed out fines for public drunkenness and drug use and testing facilities were also being set up, the prefecture said.
There were several young children among the party-goers, and a mobile clinic and Red Cross personnel were sent in to ensure everyone's safety.
With France in the grip of a heat wave, authorities handed out water and distributed face masks and sanitising hand gel.
There has been increasing concern in France about illegal outdoor parties during a hot summer season as coronavirus cases increase, especially among people in their 20s.
A seaside DJ set organised by the Mediterranean city of Nice last month also prompted health fears.