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" generally applies only to gatherings organized via telecommunications, social media, viral emails, Twitter/X, and the like (not to events organized by public relations firms or as publicity stunts).