Police sent in investigate as hundreds of illegal ravers gathered in Suffolk withdrew after they were threatened.
Police were threatened when they went to confiscate equipment
By Sunday morning an estimated crowd of more than 1,000 was at the site at Ramsholt near Bawdsey for the rave.
Officers were sent to the scene on Saturday night after people living nearby complained, but backed off after threats from some in the crowd.
Insp Terry Byford said officers were threatened and intimidated and felt they had to withdraw.
At its height there were about 500 vehicles and upwards of 1,000 people there. Numbers were down significantly on Sunday.
On Sunday afternoon she said police had been monitoring developments during the day and the organisers had been told of their obligations with health and safety and public order.
"We do have powers, but it is about matching those powers and using them appropriately because we need to protect the public, those attending the event and also the police officers," she said.
"When police were called to Ramsholt at 2230 BST on Saturday night there were already a large number of vehicles on the site and while officers did try to stop vehicles trying to reach the site they were threatened and intimidated and had to pull out.
Hundreds of the revellers set up an illegal camp
"If we can get there first we can prevent these events a lot of the time but if they are up and running it is very difficult to stop them because of the sheer numbers."
Insp Byford said police would continue to monitor and assess the site and if it was appropriate officers would go in to close it down and seize the equipment if that could be done safely.
She said though there had been no arrests on Saturday or Sunday, those connected with the rave and its organisers could face prosecution.
She added the local community, particularly the farmers, had been very understanding and supportive.