R. Kelly -- Boots Rowdy Fan from Concert ... By Singing!

When R. Kelly saw two ladies fighting over a seat at one of his concerts ... he didn't scream for security -- he SANG for security ... and it was all caught on tape.

Kelly was in the middle of his show at the Nokia Theater in Downtown L.A. this weekend ... when he noticed two female fans getting physical in the crowd. One source tells us one woman had accused the other of stealing her seat ... and they were thiiiis close to trading blows.

And that's when R. saved the day ... stopping his show ... and breaking out in song: "SECURITY ... SEC-UR-ITY."

It's hard to hear exactly what Kelly said after that ... but he definitely barked some orders to the crowd control staff and dropped a couple of cuss words ... while staying in tune.