Katy Perry hit the stage for a surprise performance at a star-studded NYC event on Thursday night ... and she pretty much blew the roof off the place.
The singing sensation was all glammed up in a ruffled cream gown and bejeweled heels as she arrived to celebrate the reopening of the Tiffany and Co. flagship store in Manhattan.
Katy was followed inside the iconic Fifth Avenue building by a bevy of bold-faced names. Among them were Hailey Bieber, Blake Lively, Florence Pugh, Zoe Kravitz, Tiffany Haddish, Mark Wahlberg, Gayle King, Pharrell Williams, Gabrielle Union and her hubby Dwyane Wade.
Everyone was having a great time, chatting and mingling around ... but the real fun started once Katy changed into a sheer gray dress and took the stage for her unexpected show. We're told Katy played 4 songs, including "Firework," "Dark Horse" and "Teenage Dream."
Check out video, obtained by TMZ, showing Katy belting out "Dark Horse" while doing a sexy dance knee-deep in a dense fog from smoke machines. As you can hear, the crowd ate it all up, begging Katy for more.
After Katy was done dazzling on stage, she posed for some selfies with fans.
Meanwhile, Hailey Bieber also grabbed some of the spotlight ... we filmed her hugging a friend and sitting back down with people at a small table. Her very famous spouse, Justin, was nowhere in sight. Seems like he had the night off.