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A crazed WWE fan has been arrested after climbing into the ring during an event in South Africa today ... and blasting Randy Orton in the nuts, TMZ has learned.

The video is pretty wild -- the fan jumps into the ring and sneaks up behind Randy while the wrestler is on the turnbuckle ... and swings his arm up between Randy's legs. Security immediately jumps in and drags the fan away, but not before Randy attempts to kick the guy.

A rep for the WWE tells TMZ, "Randy was violently attacked from behind" by the fan -- who was not part of the show ... and is now in police custody.