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UFC star Urijah Faber personally witnessed the BEATDOWN doled out by ex-NBA star/Sacramento mayor Kevin Johnson ... and he's tellin' us exactly what he saw.

Faber joined the "TMZ Sports﻿" TV show (weeknights on FS1) to break down the brawl.

ICYMI, Sean Thompson, 32, was arrested for smashing a pie in the face of Mayor Kevin Johnson -- a former 3x NBA All-Star -- at a charity event Wednesday night.

What happened next, priceless.

For the pie, Thompson got two things ... his ass kicked by the mayor, and arrested.