According to Official Guinness Records,
Flashmobs are sudden gatherings of people in a public place. Participants gather at a prearranged time and engage in a pointless activity for a brief period before disbanding. The first notable flashmob occurred in May 2003 at a shop in New York, USA, organised by Bill Wasik (USA). Police had been tipped off about the event.
The second flashmob, also organised by Wasik, avoided police intervention and was much more successful. This occurred on 3 June 2003 at Macy's department store, New York, USA.
For a complete list of 2003 records, please visit 2003 Guinness Records in United States.