Chris Brown -- I'll Take Those Panties, Thank You!

When Chris Brown sings, the panties start dropping ... and TMZ has the footage to prove it.

Chris was performing at Heat Nightclub in Anaheim Saturday ... when a very generous female fan handed the singer a silky white thong as a present.

Unclear, from the footage, if it was used or not -- but witnesses on scene tell us the club was super hot and sweaty, sooooo ...