Katy Perry could really hear her fans 'Roar' last night ... probably screaming out in horrible pain -- 'cause one of them touched a transformer and ended up on fire, and nearly disfigured the pop star ...

Katy Perry is looking ... her hit single, “Roar.” She was soon joined by Lenny Kravitz, who played guitar during “Dark Horse” and “I Kissed a Girl.” Perry was also joined onstage ...