First they boo'd the team ... then Pats fans FOUGHT EACH OTHER after Tom Brady's squad lost to the Chiefs -- and it's all on video.
It was peak embarrassment at Gillette Stadium where even the TV announcers calling the game clowned Pats fans for having the audacity to BOO while the team was down at halftime.
But it got even worse AFTER the game, when a couple of Pats fans in the disappointed crowd turned on each other and began throwing punches right in the stands.
At one point, one of the combatants ate TWO left-handed bombs to the face .