Lil Wayne -- Security Decks Fan with Brutal KO Punch (VIDEO)

Lil Wayne's security viciously attacked a fan who mocked Weezy with his iPhone .... and it was all caught on video.

Wayne was scheduled to show up at midnight on Saturday to play a show at Revolution Live in Fort Lauderdale, FL ... but the crowd became agitated when Wayne didn't get there until 3:30 AM.

According to people at the show, one fan used his iPhone to show off a scrolling message to the stage. Before Weezy arrived, the message said "Shut the f*** up and put on Weezy" ... but later it just read "F*** Weezy."

Wayne was spotted by the New Times Broward-Palm Beach pointing out the fan and sending one of his security people to deal with him.

We're told security carried the victim out of the club, but the club says no paramedics were called.