UFC Star Urijah Faber -- I Witnessed Kevin Johnson Kick Ass ... Here's What Happened (VIDEO)

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.

Watch Urijah describe the fight ... and its aftermath!