Sylvester Stallone -- Busts Up Brawl on the Set of 'Creed'

EXCLUSIVE

Rocky's not taking any mess on his set -- Sylvester Stallone had to bust up a very real brawl during filming of the latest Rocky flick, "Creed" ... TMZ has learned.

Sources close to production say things went ballistic in Ashton, PA Monday when boxer/actor Tony Bellew -- who's playing Pretty Ricky -- dove out of the boxing ring, mid-scene, and began unloading on the guy playing his corner coach. It was NOT part of the scene.

Sly still has the eye of the tiger -- we're told the fight was broken up when he, and a few others, jumped in to pull the men apart.

It's not clear what prompted the fight, but the we're told security threw everyone out and police were called to the location.

Unclear at this point, if anyone plans to press charges.