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L.A. Lakers player Lou Williams was NOT the aggressor in a Vegas fight that landed him in handcuffs, he was the VICTIM ... this according to the Las Vegas Metro Police Department.

TMZ Sports broke the story ... Williams was placed in handcuffs and slammed up against a police car in front of a Vegas casino in September -- while trying to tell cops he did nothing wrong.

Now, the LVMPD tells us ... cops had responded to a call about a fight at that location -- and when they arrived, the situation was heated, and they detained Williams while they investigated.

A short time later, cops say they learned Williams was in fact a victim -- so they let him go. Williams told police he did NOT want to press charges.