A Euro-MP has accused Sussex Police of intimidating squatters by allegedly filming them and their camp without their permission.
Dr Caroline Lucas said the police filmed tree-top protesters at Titnore Woods, near Worthing, after entering their camp without a search warrant.
The protesters are fighting plans to cut down 210 trees in the ancient woodland in Durrington to make way for a Tesco store and 875 new homes.
Dr Lucas said: "It is simply unacceptable for the police to take the law into their own hands."
And she warned police that: "We in the European Parliament are watching and will lodge a formal complaint if the laws of the land or the rights of the protesters are violated during any eventual eviction."
Eight weeks ago the protesters claimed squatters rights and built a network of connected tree houses.
They argue there are at least 875 empty homes across the borough and that another supermarket is unnecessary.
They said that on July 19 and 25 police ignored requests to leave the camp and insisted on filming them and their homes without a search warrant.
The landowners have since obtained a High Court order to evict the squatters but they are appealing against the ruling.
Dr Lucas said: "The camp and the police have enjoyed relatively good relations over the past couple of months but these latest incidents have soured that relationship and increased the likelihood of violence during any future eviction.
"The law is there to protect citizens from the state, not just from each other, and we know that terrible mistakes happen.
"I am truly worried that the alleged police law-breaking has increased the risk of violence during any future eviction.
Chief Inspector Peter Mills said: "It is important that police enter the site since we have a duty of care regarding health and safety. Should a protester get injured, emergency services need to know what is there.
"When police enter the land they do not enter any area regarded as premises.
"Police did enter an area which looked dangerous and had the appearance of a newly constructed concrete tunnel. Although attempts were made to block officers from videoing, they did so. The site visits will continue for the safety of all concerned."
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