Katy Perry -- Prank Terrorizes Real 5-Year-Old's Birthday Party [VIDEO]

Katy Perry left children in tears at a recent birthday party where she dressed up like a clown, attacked some guy with a piñata stick, and staged a scary car accident ... all for a music video -- and TMZ has video from the scene.

The hidden camera gag was shot at Knollwood Country Club outside L.A. earlier this month for Katy's "Birthday" video -- Katy showed a little preview on "Jimmy Kimmel Live" Monday night ... but her clip didn't include the crazy fallout.

Eye witnesses tell TMZ, Katy pranked a REAL 5-year-old's birthday party ... and most guests had no idea they were getting punk'd. Children were crying. One woman stormed away, visibly upset.

Check out the video ... at one point a little girl says, "Mommy, I want to leave."

We know the feeling, kid.