Aaron Hernandez -- Alleged Gun-Toting Video ... From Day Of Odin Lloyd Murder

It could be the closest thing to a smoking gun ... prosecutors just played video in court showing Aaron Hernandez holding what they claim is a firearm in the hours after he allegedly killed Odin Lloyd.

Prosecutors say the footage was taken inside Hernandez's Massachusetts house on June 17, 2013 ... showing the forming NFL star clutching what appears to be a glock firearm.

Officials believe Lloyd was shot 6 times earlier that morning ... and are convinced Aaron pulled the trigger using the very same gun in the surveillance video.

Aaron's lawyers are calling the video into question -- saying the object in Hernandez's hand could be anything from an iPad to a cell phone to a remote control.