The waters of Rome's Trevi Fountain, one of Italy's top tourist attractions, turned red on Thursday after a man dumped dye into the main pool, a city official said.
Police detained the man responsible, a statement from the mayor's office said, as city officials assessed if there had been any damage to the 18th century landmark.
The fountain, where tossing a coin into the pool is supposed by superstition to guarantee one's return to the eternal city, is a big tourist attraction in Rome. Actress Anita Ekberg famously frolicked in the fountain's pool in the 1960 film "La Dolce Vita".
"Acts like this demonstrate ignorance and a total lack of civic sense," Deputy Mayor Luca Bergamo said.
Italian media said the culprit was the same person who dyed the fountain red in 2007 to protest the cost to the city of hosting an international film festival.
Thursday's action took place on the opening day of Rome's 2017 film festival.