First coined in 2003, the term ‘flash mob’ is used to describe a large group of people who assemble suddenly in a public place. Before quickly dispersing, the flash mob participants are known to perform a particular action for a brief period of time. In recent history, such mobs have been organized via telecommunications channels, social media, or viral emails.

Interestingly, the first flash mobs were orchestrated for apolitical purposes.

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