Avon and Somerset Police were called to a disused farm building near Hambrook, South Gloucestershire, to shut down an unlicensed rave in the early hours of Sunday morning.
400-500 reportedly attended the event, with officers alerted to an unusually large number of vehicles entering the area on Saturday night.
Posting on Twitter, Avon and Somerset Police said: “We received a call from a member of the public at about 12:05AM this morning (Sunday 30 October) informing us vehicles had started arriving in the Old Gloucester Road area of Hambrook.
“Approx 100 people were on site at about 10:AM but have now left. Officers remain in the area,” they added.
Chief Inspector Dan Forster told the BBC: “For a police officer to walk in and just pull the plug when you’ve got 500 people in an unlit warehouse is a really unsafe thing to do.
“This was an unlicensed an unmanaged event so you don’t have the security features that you would do with a festival type event,” he continued. “The decision was taken to try to prevent more people from attending, to make a sound assessment and then decide when we could safely shut that event down.”
According to BristolWorld reporting, some attendees had come from as far as Essex to attend the event.

