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UFC legend Matt Serra claims some drunk a-hole was threatening the servers at a Vegas cafe on July 4 ... so, he took action -- AND TOOK THE GUY DOWN!

Serra was chllin' at the Grand Cafe at Red Rock Casino -- the night before he's set to be inducted into the UFC Hall of Fame -- when he saw the alleged a-hole being a jerk to staff.

The former welterweight champ subdued the patron -- while the guy continued to yell at Serra, "I'm gonna f*ck you up."

Spoiler: he did NOT "f*ck" Serra up.

Security rushed to the scene and took over.