Over 20,000 revelers gathered for an illegal rave on a military site in central France despite the risks of unexploded ordnance nearby.

The event organized on social media as the “Bourges Teknival” has been taking place on a military firing range which authorities have deemed “very dangerous.”

Partygoers from across Europe began arriving Thursday at the vast site which has been used to test canons, missiles and shells.

The head of the local prefecture, Philippe Le Moing Surzur, said over 2,000 cars had streamed onto the site for the event, which was classified as illegal early on Friday morning.

After organizers failed to seek authorisation for the event, 600 police officers were deployed on Friday, issuing dozens of fines mainly for the possession of narcotics and driving offences. Emergency services have treated at least 12 people for minor injuries, according to the prefecture.

The local authority advised people throughout to avoid travelling to the site, especially to wooded areas which contain remnants of unexploded ordnance which have been deemed “high risk” areas.

In a mock prefectural decree circulated on social media, organizers said the event was being held to protest against a new law which has been tabled by French authorities seeking to “strengthen the criminalization of rave parties.”

The French movement promoting illegal raves, referred to as “free parties” dates back to 1993 when a Teknival was organized outside Paris in Fontainebleau, drawing thousands of ravers.

Since then, illegal raves have been held in abandoned fields or spaces across the country, mimicking the rave culture popularised in the United Kingdom from the late 1980s.

Organizers expected as many as 30,000 people from France and neighboring countries to arrive at the peak of the event this weekend. It comes as French parliament is seeking to tighten legislation against unauthorized rave parties, introducing prison sentences for organizers and fines for attendees.

“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.