The brawl occurred as patrons watched the fight between UFC champion Conor McGregor and undefeated boxer Floyd Mayweather Junior.
Shocking footage shows at least 10 people swinging punches during the fight in Surfers Paradise Beer Garden in Queensland, Australia.
Other patrons are shown clapping and filming the fight on their phones, as cheers and whoops ring out in the pub.
Later the chaos spilled out onto the streets outside, with police forced to tackle several people to the ground.
One person was shown being wheeled into an ambulance with blood streaming down his neck. He celebrated with onlookers before flipping the middle finger.
One witness told 9News: “One guy started the fight with the two brothers. Four guys ran in and the fifth guy ran in trying to hit the guys defending themselves with the glass but he missed and hit his own friend in the forehead.”
Several people were injured in the chaos.
A second witness said: “All I see is this bloke come up with blood all over him, like he’d been stabbed.”
A third witness said: “To be honest I just think it’s stupid. Everyone’s alcohol-fuelled and think they can just pick a fight.”
Police confirmed a stabbing had taken place at the venue.
Jim Plowman of Queensland Police said: “A 24-year-old male produced a knife and stabbed a 29-year-old victim in the neck.”
The chaos occurred as McGregor and Mayweather went head-to-head in the most-anticipated boxing fight in years took place in Las Vegas.
McGregor, making his professional boxing debut, shocked critics by competing with 49-0 Mayweather in the early rounds before weakening as the contest wore on.
After Mayweather delivered an eye-watering string of punches in Round 10 the fight was ended by the referee for McGregor’s own safety.