Campaigners from advocacy group No Dash for Gas staged a weekend sit-in at Balcombe, a small village in West Sussex. Energy company Cuadrilla Resources is in the early stages of oil exploration there and environmental groups fear the campaign is a prelude to a hydraulic fracturing, or fracking, operation.
Cuadrilla last week scaled back operations at Balcombe on the advice of local police. Campaigners stormed the company's headquarters early this week.
More than 20 protesters were arrested by West Sussex police for blocking the drilling site in Balcombe. No Dash for Gas said local police "acted in a provocative way" and roughed up some protesters during arrest.
Campaigner Amy Scott said the group filed a complaint after a video broadcast by the BBC showed an officer "violently" arresting a protester. The group said the officer was not wearing an identification number.
"The tactics and force used was completely disproportionate to the calm and friendly situation," Scott said in a statement Monday.
Sussex police said 25 people were arrested for disturbing public order or blocking a highway. Two men were arrested on suspicion of assaulting a police officer and a 17-year-old girl was arrested for "possession of an offensive weapon."
Cuadrilla had no update on the situation. It said it would require additional permits should it decide to advance with a fracking campaign at Balcombe.