A group called Ultima Generazione or Last Generation have poured what they described as a charcoal-based black liquid into the water of the Barcaccia fountain at the base of the Spanish Steps in central Rome.
The group posted a video on Twitter, showing three men and a woman inside the fountain opening paper bags of a black powder.
“It is absurd that this gesture should shock you, when we are experiencing a drought emergency that is putting agriculture, energy production in crisis,” the group said in the tweet.
The group was stopped by Carabinieri officials and Rome Capital police on Saturday, a police spokesperson said.
They are in custody pending charges of defacing a public monument and entering a fountain.
The water remained black with visible stains to the marble fountain on Saturday afternoon.
Rome’s mayor Roberto Gualtieri visited the fountain – designed by Pietro Bernini in 1629 – and posted a photo of himself staring at the inky water.
“Rome is at the forefront in the fight against climate change and in the protection of the artistic heritage,” he said.
“Throwing black liquid into the Barcaccia, risking ruining it, is an absolutely wrong gesture that does not help the environment,” he added, saying work was under way to ensure there was no permanent damage.
The same group glued themselves to a plinth in the Vatican museums, to Sandro Botticelli’s “Primavera” masterpiece at the Uffizi galleries in Florence and to the Unique Forms of Continuity in Space statue at the Museo del Novecento in Milan last year.
They are also facing charges in Rome for throwing orange paint on the Italian Senate façade in January this year.