In the clip, obtained by TMZ Sports, you can see several men were out at Eastern Market in downtown Detroit following the Lions' loss to the Tampa Bay Buccaneers at Ford Field ... when an argument breaks out.

At least half a dozen people can be seen exchanging words ... when, suddenly, one of the guys wearing a black T-shirt and black jeans brandishes a gun in his right hand.

After appearing to cock the firearm, the man walks into the face of another taller man in red shorts. Moments later, that guy then pulls out his own gun -- and shoots one time.

Nearly everyone in the area disperses after the shot rings out ... and minutes later -- the video shows two men were left bleeding and laying motionless on the ground.

Authorities say the bullet that was fired ended up passing through both men during the incident -- causing fatal injuries to the two.

Cops initially placed the shooter in custody -- although later in the week, officials revealed he would not face criminal charges because he "was defending himself."

"We looked at all the applicable laws," Wayne County Prosecutor Kym Worthy said, "and there is no crime that can be charged or proven beyond a reasonable doubt."

Eastern Market officials say they've canceled their planned tailgate for the Lions' next home game on Sept. 30 in wake of the tragedy.