(CNN) — Environmental protesters have sprayed Britain’s most famous prehistoric structure, Stonehenge, with orange paint a day before huge crowds are expected to gather there for the summer solstice.

A video posted on X by Just Stop Oil shows two of the group’s activists spraying the landmark using fire extinguishers.

“The Just Stop Oil protestors demanded the incoming government sign up to a legally binding treaty to phase out fossil fuels by 2030,” the group posted on X.

The action took place around 12 p.m. local time (7 a.m. ET) at the ancient site near Salisbury in the southwest of England, Wiltshire Police said in a statement. Police have arrested the two protesters “on suspicion of damaging the ancient monument,” the statement said.

The official Stonehenge X account described the incident as “extremely upsetting.”

“Orange powdered paint has been thrown at a number of the stones at Stonehenge. Obviously, this is extremely upsetting and our curators are investigating the extent of the damage,” the post said. The site remains open, according to English Heritage, which manages the site.

The protesters were a 21-year-old student from Oxford and a 73-year-old man from Birmingham, Just Stop Oil said in a press release.

In anticipation of people meeting at Stonehenge on Thursday to mark the Northern Hemisphere’s longest day of sunlight, English Heritage has published “conditions of entry.”

“Stonehenge is a World Heritage Site, a Scheduled Ancient Monument and is seen by many as a sacred place. We ask that all those attending respect it and those celebrating around it,” the website said.

Responding to the incident on X, British Prime Minister Rishi Sunak wrote: “Just Stop Oil are a disgrace.”

The same group made headlines last month when two protesters smashed the glass protecting the Magna Carta, a famous British manuscript from the 13th century, at the British Museum in London.

Climate activists have been staging increasingly high-profile protests, many of which have involved attacking high-value artworks including the Mona Lisa and Van Gogh’s “Sunflowers.”

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