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Chumlee from "Pawn Stars" was involved in a crazy -- and bloody -- fight right in the middle of Hollywood Blvd. this weekend ... but the reality star tells TMZ it was all in self-defense.

A video has surfaced showing Chum and his pals hanging out in front of Chum's Rolls Royce ... when they are approached by an unknown man.

Chum tells us ... the guy was harassing the group and demanding a ride in the Rolls. When Chum's group refused, Chum says the man threatened to pull a gun.

Chum says the man then reached for something in his pocket ... so his boys took action, socking the guy in the face HARD ... dropping the man to the pavement.

Once the guy hit the floor, Chum and his crew scrambled to get back in the car and quickly fled the scene ... while the man tried to pick himself up, blood pouring out of his face.

Chum tells TMZ, "Instinct just took over. [The man] didn’t even know I was on TV or anything … just wanted to ride in our car and start problems.”