flash mob

a.k.a. flashmob, smart mob, flash crowd

What is a flash mob? It's a large group of people who gather together suddenly in a public place, to do something for a brief amount of time, and then quickly disperse.

The term "smart mob" is generally applied only to gatherings organized via telecommunications, social media, viral emails, Twitter, and the like (not to events organized by public relations firms or as publicity stunts).