Violence flared as police tried to break up an illegal rave involving up to 600 revellers on farmland in Essex.
Police clearing the site on Sunday morning
Police said an officer suffered a severed fingertip, a patrol car was burnt out and missiles were thrown during clashes at Great Chesterford.
Nine officers were hurt, with injuries including head wounds and a leg wound, Essex Police said.
Two revellers suffered head injuries, and 35 were arrested on suspicion of conspiring to cause a public nuisance.
Describing what he saw, one reveller told BBC News that the police were in riot gear and were "heavy-handed".
"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," he said.
He claimed that one person had an eye injured during a clash.
Essex Police did not respond to the claims. An Essex Ambulance Service spokesman confirmed that two party-goers were taken to hospital, one with an eye injury.
At one point 40 revellers attempted to cross the nearby M11 motorway on foot, police said.
Emergency roadblocks were erected and the motorway speed limit reduced to 40mph.
One lane of the M11 was kept closed for much of Sunday which resulted in tailbacks stretching more than ten miles at one point.
Forces from Essex, Suffolk, Cambridgeshire, Norfolk and Hertfordshire were involved in the operation.
The landowner had not given permission for the party and it had no entertainment licence.
An Essex Police spokeswoman said police were still clearing the site on Sunday morning.
She said: "Police have powers under the Criminal Justice and Public Order Act to take positive action against such events.
"Public safety and upholding the law were paramount, so Essex Police mounted a controlled operation throughout yesterday to clear the site.
"This involved initially blocking the entrance to the farm to ensure that no more people joined the event."
The officer with the severed fingertip was treated at Broomfield Hospital in Chelmsford, while two others were taken to Addenbrooke's Hospital in Cambridge.