A Lions fan lost his life on Sunday -- after he was shot and killed during a party after Detroit's home game against the Buccaneers.
Detroit Police say the man -- who was in his 40s -- had been at a tailgate at Eastern Market in downtown in the hours after the Lions lost a heartbreaker to Tampa Bay ... when he got into an altercation with another man.
Cops say witnesses told them the two guys squared up as if they were going to fight -- when, suddenly, the man's opponent pulled out a gun and fired.