One of Italy’s biggest tourist attractions was vandalised in a demonstration against corruption.
Deputy Mayor Luca Bergamo said: “Acts like this demonstrate ignorance and a total lack of civic sense.”
Millions of tourists head to the 18th century landmark every year.
It is tradition to throw a coin into the pool, with a superstition it guarantees return to the eternal city.
Actress Anita Ekberg famously frolicked in the red fountain’s pool in the 1960 film “La Dolce Vita”.
The famous fountains had to be switched off for emergency checks and cleaning.
Artist Graziano Cecchini was behind the stunt, which came 10 years after he did the same thing in protest of the cost to Rome of hosting the international film festival.
Thursday’s action took place on the opening day of Rome’s 2017 film festival.
Mr Cecchini said in a statement prepared before his arrest: “In 10 years a few things have changed, many of them worse.”
He added: “Rome has always been the mirror of the country and it is now asleep in the midst of its dirt and corruption. I have always believed in the forms of art that excite and communicate astonishment, feeling, reaction. Today the capital needs to react, to wake up from this numbness by shouting ‘Not anymore!’.
“After 10 years of my previous performance to try to shake some souls, I accept the risk of raising my voice for all those who think that they ‘don’t achieve anything at all with that’.
“Instead, the Trevi Fountain returns to shine in red to shout that Rome is not dead, it is alive and ready to be the capital of art, life, rebirth.”
No damage to the fountains is reported, it is understood.