Lincolnshire Police have confirmed that three officers were treated at hospital after being injured at an illegal rave.
A further 18 officers reported minor injuries which did not require hospital treatment after the incident at Twyford Woods, near Bourne.
Four people were arrested last night at the event and a further 12 arrests have been made since then for various offences relating to the illegal rave.
Police remain at the site and are denying entry to the area and will continue to do so.
It is estimated that about two hundred people remain at the site.
One lane of the south bound A1 remains closed to ensure the safety of people walking away from the site and returning to their cars which were left alongside the road.