EXCLUSIVE
It all went down Friday night at Papa Juan Cigar Room in Harlem ... when Kelly showed up with his entourage -- and a bunch of the bar patrons approached him.
One of the guys began recording R.K. with his camera phone -- and that's when Kelly jacked the guy's cell ... and recorded his rendition of "Chestnuts Roasting."
The reaction from the bar crowd is pretty awesome -- check it out.
