This incident was reported in the Washington Post, CNN, and other outlets, including TMZ, on Monday morning. It is video of Trump supporters dragging a law enforcement officer by his helmet (unclear whether he is Capitol police) down the marble steps leading into the structure, and beating him over and over, at least one of them using an American flag.
From TMZ’s report:
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!”