Part of Stonehenge has been covered in orange powder paint by protesters.
Two Just Stop Oil campaigners sprayed the paint on the historic site near Salisbury, Wiltshire, on Wednesday.
The move comes the day before celebrations beginfor the Summer Solstice at the 5,000-year-old landmark.
Wiltshire Police confirmed two people had been arrested on suspicion of damaging the ancient monument.
Just Stop Oil said the orange powder was cornflour and claimed it would "wash away with rain".
Members of the public were heard shouting "no" and running to intervene as the campaigners ran up to the stone circle at around 12:00 BST.
A spokesman for Wiltshire Police said: "We responded to a report that orange paint had been sprayed on some of the stones by two suspects.
"Our enquiries are ongoing and we are working closely with English Heritage."
