Check out this gruesome clip of insurrectionists breaking into an opening on a side entrance to the building, which was being guarded by a line of cops in tactical gear ... one of whom ended up laid out on the ground, face first ... right at the feet of rioters.

The crowd showed him no mercy ... immediately trampling him, bludgeoning him with objects and projectiles, dragging him down the steps they were storming -- pretty much having their way with his limp body as his colleagues tried pulling him away.

There's even an overhead shot of the scene, and it's absolutely surreal. You hear someone off-camera yell, "Get that motherf***** out of there! Take him out!"

Now, there've been rumors this officer might've been the one who was killed -- Officer Brian Sicknick, who died from injuries he sustained during the siege -- but that has NOT been confirmed. Some social media users say this officer is actually with Metro D.C., not Capitol Police, but again ... we haven't been able to confirm.