Champion Khabib Nurmagomedov ended Irish fighter Conor McGregor's attempt to win back the UFC lightweight title when he sealed a brutal submission victory with a rear naked choke hold in the fourth round on Saturday.
But the Russian champion then exacerbated several months of hostilities between the fighters' camps. Nurmagomedov stepped away from the prone McGregor and immediately pointed at the Irishman's corner, shouting and throwing his mouthpiece.
The men in McGregor's corner appeared to respond with taunts, and Nurmagomedov climbed over the fence and fought with Dillon Danis, a Bellator welterweight who trains with McGregor.
Meanwhile, two men entered the cage and sucker-punched McGregor, who defended himself before security personnel separated everyone.
Nurmagomedov and McGregor both left the ring before the championship belt could be put around Nurmagomedov's waist. UFC President Dana White said he feared a melee in an arena dominated by fans of McGregor.
Both fighters were subsequently escorted from the arena by police and security.
No belt was presented and no speeches made, but the ring announcer declared Nurmagomedov the winner to a chorus of boos of the crowd.
White later confirmed three members of Nurmagomedov's team had been arrested over the incident.
McGregor, making a comeback to the octagon after nearly two years away, had looked dangerous in the early striking exchanges but as expected, Nurmagomedov quickly took the fight to the ground.
The Russian smothered him with his wrestling skills and prevented him from getting back to his feet for the rest of the round.
The Irishman was then wobbled by an early right cross from the Russian in the second round and the fight was soon on the floor again, but this time the Russian champion was able to keep the challenger from getting his back to the cage.
McGregor survived until the end of the round, giving a tired smile as the horn went, and in the third period he managed to keep the fight standing, tripping off shots to the head and body as he rediscovered his range.
However, it proved to be too little, too late, and the Nurmagomedov was once again on top in the fourth, sapping McGregor's energy with his excruciating pressure before the choke ended the fight 3:03 into the fourth round.
The victory stretched his professional record to 27-0 and handed the Irishman his second defeat in the UFC.