Around 20,000 partygoers gathered on Saturday for an illegal rave at a military site in France, defying warnings about the risk of possible explosions from old ordnance.

Thumping techno music echoed across a vast firing range near the central French town of Bourges as revelers - many dressed in brightly coloured outfits, others shirtless - assembled for the unauthorized event, known as a "free party," which began on Friday.

Behind them, tents and vans stretched across the grassy field.

Organizers expected as many as 30,000 people from France and neighboring countries to arrive at the peak of the event this weekend.

The get-together comes as the French parliament is seeking to tighten legislation against unauthorized rave parties, introducing prison sentences for organizers and fines for attendees.

'Extremely dangerous'

"Despite its illegal nature, the government has mobilized to ensure the safety of this event and limit any disturbances," the prefecture said.

Philippe Le Moing Surzur, the local prefect, said the site was "extremely dangerous due to the unexploded ordnance it may contain."

While modern shells posed no threat, he warned of the risk from older unexploded ordnance dating back to World War II.

He said that even the France's Directorate General of Armaments, which owns the land, did not have precise knowledge of the risks.

"This is a site that has been in use for 150 years, and we know there are potentially old artillery shells there," he said, adding that bomb disposal experts discover them regularly.

The firing range covers 10,000 hectares (25,000 acres) and is crisscrossed by roads that are closed during tests but open to the public at other times.

Signs prohibit access to the range itself, but it is not fenced.

The organizers have urged attendees to refrain from lighting fires, digging or picking up any objects.

'Unmanageable' event

The prefect said some 2,000 cars arrived unannounced on Thursday night.

"It's unmanageable when you don't know in advance," he said.

To prepare for any eventuality, authorities designated helicopter landing zones, he said.

"This is clearly the only solution, since I don't have the means to forcibly remove 20,000 people from this site."

Emergency services treated 12 people with minor injuries on Saturday morning.

The prefect said one person was struck by a vehicle, and a number of the injured people cut themselves while handling sharp shrapnel.

Revelers see the giant party not only as a chance to unwind, but also as a protest against the rave-curbing legislation.

"It's a way of showing that participants are mobilized and will keep coming, whatever happens, and will continue to challenge these laws," a member of the Tekno Anti Rep collective said.

Anti-establishment culture

A free party is usually an illegal form of rave rooted in anti-establishment culture.

They are held without permission in remote locations, and admission is free or by donation.

"I had heard there were going to be a lot of people, but once you're there, it's still a shock to see so many people gathered in one place," a 22-year-old man said on condition of anonymity.

Another partygoer, 19, also anonymous, said the turnout was "a strong message against repression."

Around 600 police and 45 firefighters have been deployed at the site.

"We're getting ready for a big get-together tonight," said Edith Raquin, the Mayor of Cornusse, a village of 220 people located less than two kilometers from the site. 

She said that revelers came across as "peaceful people" and some elderly residents were "delighted" to talk with them.

"They are polite, they say hello," Raquin said.