The Arlington PD has opened an investigation into a disturbing video showing a Carolina Panthers fan being choked out at the Dallas Cowboys game by an AT&T Stadium security guard ... TMZ Sports has learned.
The video went viral over the weekend ... showing a security guard trying to detain a man in a Panthers jersey who was reported as "disruptive."
The security guard eventually puts the man into a tight headlock ... and several concerned fans jumped in trying to loosen his grip.
The fan -- John Small -- filed a police report on Monday morning and now we're told cops are actively investigating the situation as a possible Class A misdemeanor assault.
Small insists he did NOTHING WRONG and is asking other fans to come forward with video if they have it.