Seven young activists protesting against climate change climbed into the Trevi Fountain in Rome on Sunday and poured diluted charcoal into the water to turn it black.
The protesters from the "Ultima Generazione" ("Last Generation") group held up banners saying "We won't pay for fossil [fuels]," and shouted "our country is dying."
Uniformed police waded into the water to take away the activists, with many tourists filming the stunt and a few of the onlookers shouting insults at the protesters, video footage showed.
In a statement, Ultima Generazione called for an end to public subsidies for fossil fuels and linked the protests to deadly floods in the northern Italian region of Emilia-Romagna in recent days. The group said one in four houses in Italy are at risk from flooding.
Rome mayor Roberto Gualtieri condemned the protest, the latest in a series of acts targeting works of art in Italy.
"Enough of these absurd attacks on our artistic heritage," he wrote on Twitter.
The tradition is for visitors to toss coins into the famous 18th century Trevi Fountain to ensure that they will return to Rome one day.