Ugly scenes marred UFC 229 as lightweight champion Khabib Nurmagomedov began a brawl by the octagon after beating Conor McGregor.
Seconds after landing a fourth-round submission, the Russian vaulted the cage and headed towards Irishman McGregor's team, prompting a melee.
Meanwhile, McGregor was involved in a bust-up in the octagon with members of his opponent's team.
Three of Nurmagomedov's party were arrested but later released.
UFC president Dana White said McGregor had refused to press charges.
Earlier White could be heard telling the 30-year-old winner he could not receive his belt in the octagon as he feared it may incite trouble among those present at Las Vegas' T-Mobile Arena.
He later said the Nevada State Commission, which sanctioned the bout, has withheld Nurmagomedov's fight payment pending an investigation.
"I know if we put the belt on him [Nurmagomedov] in the octagon there'll be things thrown," said White. "I just said let's just try and get him out of here.
"Police officers were getting punched, security guys were getting punched. I actually think police and security did an incredible job. Khabib jumped over the cage like one of those Parkour guys. What are you expected to do? "
Read also:Sanchez's last-munite header completes Manchester united comeback
McGregor appeared stunned and was quickly escorted away by his team, while heavyweight and light-heavyweight champion Daniel Cormier entered the octagon to seemingly try to calm Nurmagomedov, who left the arena flanked by police.
What was billed as the biggest bout in UFC history, one set to break the organisation's pay-per-view record and where McGregor returned after a 693-day absence, will now prompt difficult questions for White and the company.
How the chaos unfolded
Nurmagomedov largely dominated proceedings before administering a rear-naked choke, prompting McGregor to tap out with one minute and 57 seconds of round four remaining.
The champion instantly shouted at the beaten man and then pointed at McGregor's team before exiting the ring and attacking Dillon Danis, a fighter who trains with McGregor.
While the brawl took place, McGregor was involved in a bust-up in the octagon with one of Nurmagomedov's training partners.
Nurmagomedov left the arena with fans throwing objects towards him, while White said he felt "sick" over the scenes.