The balls bounced down the steps, filling the boat-shaped Barcaccia fountain in the piazza below in a spectacle that stunned passers-by, who snapped photos and scooped up the balls as souvenirs.
Officials were forced to briefly cordon off the popular tourist spot. Police detained Cecchini and three others, Italian news reports said.
Cecchini, interviewed at the scene, told Italian TV newscast TG5 that he uses "art -- if we want to call it art -- to stress our malaise."
He said half a million balls were deployed.
Street cleaners later shoveled the balls into garbage trucks and hosed down the stairs to remove strays.
Following his Trevi Fountain stunt in October, Cecchini was placed under investigation for possible damage to historical or artistic buildings.