Updated, 3:49 p.m., scroll for video.

A greyish 18th-century landmark in Rome was transformed into a colorful mess today when a half-million plastic balls were dumped at the top of the Spanish Steps.

The balls bounced downhill before filling the Barcaccia fountain, leaving surprised passers-by and tourists snapping pictures of a scene that recalled the indoor-playground ball pits that spark joyful frolicking by children of all ages.

Many people collected some of the balls and took them away as an “unforgettable souvenir of this event,” which was pulled off by Graziano Cecchini, the man who turned Trevi Fountain blood-red last October.

“It is an artistic operation that documents with art the problems that we have in Italy,” Mr. Cecchini said at the scene. He was later detained by police, news agencies reported.

Amid the excitement, several shopkeepers declared themselves baffled — and unimpressed.

“It was a lot of balls — and that was it,” said Gilberto Guibbini, 38, who had trouble arriving at his job at a shoe store because the stunt forced the closing of streets leading to the Spanish Steps. “I don’t know why.”

“There is nothing interesting about it,” he added. “He just wants attention.”

Update, 3:49 p.m. This YouTube video captures the prank in all its glory. Enjoy:

Update, 3:54 p.m. A colleague pointed out echoes in the world of advertising: a commercial for Sony Bravia televisions, and another for a soft drink called Tango.

Ian Fisher contributed reporting from Rome.