Around 15,000 people descended on the Piazza Del Popolo to dance to the South Korean rapper's hit "Gangnam Style."
Organizers used social networks and word-of-mouth to bring everyone together for the "Roma Flash Mob."
Participants wore white t-shirts with portraits of Psy on them during the event.
"Gangnam Style" has been at the top of Italian charts in the last few weeks.
Psy's video is also a YouTube sensation with more than 630-million views and counting.
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