A flash mob is a large group of people who get together in a public place, do something unexpected and then quickly leave. In this particular case, the dancers started out small and eventually added more and more people to create a radiating effect at the Galleria Alberto Sordi. The group ended the event with their pointer fingers and thumbs in the air, shouting, "Glee!"
Glee aired in Britain last week, so some say this flash mob was to promote the show as it makes its way through Europe. Watch what happened in the Fox video after the jump!