According to Official Guinness Records,
The most people dressed as superheroes was achieved by 1,580 participants in an event organised by Paramount Studios to promote the Dreamworks movie 'Megamind' starring Will Ferrell, Brad Pitt and Tina Fey in Los Angeles, California, USA on 2 October 2010.
For a complete list of 2010 records, please visit 2010 Guinness Records in United States.