Katy Perry apparently left children in tears when she gatecrashed a birthday party.

As part of her music video for track Birthday, the 29-year-old singer dressed up as a clown and acted as a mayhem-causing entertainer. The gag included taking a swing at a piƱata but hitting a man instead and staging a fake car accident by running out into the road.

While the whole prank was in jest, TMZ claims Katy actually ambushed a real five-year-old's birthday bash and that most of the guests had no idea what was happening.

The website posted a clip from the scene today, shot at Knollwood Country Club outside Los Angeles. In the video a woman storms away from the party in anger, while a small child can be heard saying: "Mommy, I wanna leave."

Katy gave a small preview of the music promo during a recent appearance on Jimmy Kimmel, but it cut out before the supposedly upset reactions. She joked that her undercover prank was top secret and that "nobody knows" about it.

"I was sick of making gorgeous music videos where I'm, like, sexual..." she quipped.

Host Jimmy shot back: "People are sick of that; they're really tired of looking at you!"

During her stint on the show the star acknowledged her breakup from John Mayer for the first time. The pair had been in an on/off relationship since 2012 but in February it was reported they had split again.

While speaking to a 'super fan' on the show, Katy pulled a mock crying face as she exclaimed: "At least you have a boyfriend!"