Police officers remained at the rave site the next day

Violence broke out at the event at Great Chesterford on Saturday which was attended by up to 600 people.

Essex Police said a minority of partygoers had acted "disgracefully" and it responded "proportionately".

Twenty five police officers and a number of revellers were hurt, amid complaints from some revellers that police had been "heavy handed".

One reveller told BBC News: "They came and surrounded the first sound system and then got closer and closer until it turned into a proper confrontation and they started beating people."

Hospital treatment

Essex Police said any complaints would be taken seriously and thoroughly investigated.

It said in a statement: "A minority acted disgracefully and with unprecedented aggression and violence.

"Officers are well trained and equipped and used conflict resolution tactics before deploying their personal protective equipment, including CS spray and asps (batons). "Such use would be deployed proportionately and through necessity."

The statement added that some officers needed hospital treatment for injuries caused by broken bottles or other weapons, including scaffold bars.

'We were surrounded'

"In addition a police vehicle was set on fire and police dogs were attacked," it said.

Essex Police Federation chair Sue Kelly said: "I spoke to an officer that had been there... and he works on a specialist unit that deals with this sort of thing a lot and he said it was the most frightening experience he'd ever had.

"He said 'We were surrounded, we were being pelted with missiles, things like fencing and posts and scaffold poles were being used against the officers'."

Officers from Suffolk, Cambridgeshire, Norfolk and Hertfordshire forces were also involved in the operation.

Of the 40 people arrested on suspicion of conspiring to cause a public nuisance, six have been released without charge and 34 others bailed.