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NBA YoungBoy has some rabid fans ... but one took it all too far when he went totally ballistic and beat the living piss out of an elderly man working the rapper's concert in Kansas City last weekend.

Video posted on X captured every moment of Sunday's beatdown in the crowd inside KC's T-Mobile Center during NBA's sold-out "MASA" Tour.

Check out the clip ... the young fan -- who police describe as a juvenile -- slams the guest services employee into the seats and starts using his face as a punching bag. During the brutal attack, the old man's glasses are knocked to the ground.

Another person jumps in and grabs hold of the assailant, pulling him off the victim, who reportedly suffered serious injuries that were not life-threatening. Gallery Launch Gallery Getty

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