Environmental protesters sprayed paint on Britain’s Stonehenge on Wednesday, with orange marks covering some of the stones of the world-famous prehistoric megalithic structure on the eve of the summer solstice celebrations.

Two people have been arrested on suspicion of damaging the ancient monument, local police said in a statement.

In video released by environmental group Just Stop Oil, two protesters were seen running toward two of Stonehenge’s megaliths and spraying paint as another person attempted to stop them.

The paint spraying was condemned by U.K. Prime Minister Rishi Sunak, who called it a “disgraceful act of vandalism.”

The monument in southern England is a UNESCO World Heritage Site and one of Britain’s most visited tourist spots.

Members of the public were able to walk around within the stone circle until 1977, when it was fenced off due to concerns over damage from a sharp rise in the number of visitors.

Just Stop Oil has gained prominence in Britain for disruptive environmental protests, with its activists shutting down major roads, disrupting cultural and sporting events and even throwing soup at a van Gogh painting.

The group wants the British government to end the extraction and burning of oil, gas and coal by 2030.

English Heritage, which manages the site, said it was “extremely upsetting” and said curators were investigating the damage. Stonehenge remains open, it said.

Just Stop Oil said on the social media platform X that the paint was made of cornstarch and would dissolve in the rain.