12:26 PM PT -- A spokesperson for United tells us the employee has been fired following the incident.
8:39 AM PT -- Cops tell us the passenger, ex-NFL player Brendan Langley, was arrested and charged with simple assault.
Langley was a third-round pick in the 2017 NFL Draft out of Lamar University ... and is currently playing for the CFL's Calgary Stampeders.
7:05 AM PT -- Law enforcement tells us the passenger was arrested, not the employee ... despite the passenger's claim he didn't throw the first punch.
An airline passenger and a United Airlines employee were in an all-out brawl at Newark Liberty International Airport, and the worker ended up flat on his ass.
Witnesses say the 2 began arguing before pushing, slapping and ultimately punching each other.
The video starts mid-fight ... the worker seems to push the passenger, who then slaps the employee in the face, and then all hell breaks loose.
The 2 square off and then the passenger throws punch after punch ... and the blows clearly landed, because the employee eventually falls behind the counter and clearly has trouble getting up and steadying himself.
The craziest part ... the employee is clearly dazed and bloodiest, but comes back for more, and that's where the video ends.
We reached out to airport police to find out if anyone was arrested ... so far, no word back.