A Flash Mob ?

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Flash Mob - Auckland, New Zealand

From Wikipedia:

A flash mob (or flashmob) is a group of people who assemble suddenly in a public place, perform an unusual and seemingly pointless act for a brief time, then quickly disperse, often for the purposes of entertainment, satire, and artistic expression. Flash mobs are organized via telecommunications, social media, or viral emails.

Read more here:

http://en.wikipedia.org/wiki/Flash_mob