There were disgraceful scenes in the T-Mobile Arena as a mass brawl erupted following Khabib Nurmagomedov's victory over Conor McGregor.
Nurmagomedov delivered a career-best performance and should have been well ahead on the scorecards when he mounted McGregor and locked in a rear-naked choke, forcing the Irishman to submit three minutes and four seconds into the penultimate round.
Nurmagomedov therefore retained his lightweight title but then threw his gumshield, mounted the cage and appeared to attack someone from McGregor's team.
A prone McGregor was then seemingly attacked by someone from Nurmagomedov's camp as the bad blood that has been building up ahead of this fight came to a head.
UFC chief Dana White confirmed later that three of Nurmagomedov's team were arrested and taken to jail but McGregor will not be pressing charges.
McGregor looked crestfallen by the defeat but did not seem to be harmed by the attack as he exited the cage to raucous cheers from a crowd largely favouring the Irishman.
Nurmagomedov was showered with boos and was spoken to by UFC president Dana White before exiting the arena to yet more jeers.
In the run up to the fight, McGregor launched a series of scathing insults at Nurmagomedov, including one incident where he said "the smell of s***e of your dad".
McGregor did not attend the post-fight press conference but Nurmagomedov made a brief appearance.
"What's up guys. I want to say sorry to the athletic commission, sorry to Vegas," said Nurmagomedov.
"This is not my best side, I'm a human being and I don't know how people can talk about me jumping out the cage.
"What about him talking about my family, my religion, the bus? Why are people talking about me jumping over the cage? My father talks to me about respect."
"They call him a two-time champion, two-weight world champion but he taps," he added.
"People, the media change MMA. This is a respect sport, not a trash talking sport. Guys, you cannot talk about religion. This stuff is important.
"I know my father is going to smash me when I go home. Nevada, sorry, Vegas, sorry. Thank you so much, I am very proud.
"Putin just called me and told me he's very proud of me, he said congratulations. I said everything was going to change on October 6th."
After the fight, UFC President Dana White revealed that three members of Nurmagomedov's team have "been taken to jail" but McGregor will not be pressing charges.
White said: "Three of his guys have been arrested, I believe they're being taken to jail right now."
Upon being asked what he could see from his cageside seat immediately after the fight, he replied: "A lot of bad stuff.
"A lot of stuff that shouldn't happen, Khabib jumped over the cage and went after one of Conor's guys who I guess was trash talking him.
"I don't know what to say right now, I'm disgusted."
White also revealed that McGregor was unharmed after being assaulted by a member of Nurmagomedov's team immediately after the fight.
"I just talked with him, he's okay," he added. "He's upset about the fight not the fight after the fight."
White confirmed Nurmagomedov's pay cheque has been withheld but McGregor will receive his $3m purse. Nurmagomedov may also be stripped of his belts.
A downbeat White added: "I don't give a shit how many pay-per-view buys we did. I don't give a shit right now. I swear to god, from the bottom of my heart, on my daughter's life, I don't give a shit."
But he refused to agree that his handling of the bus attack in Brooklyn earlier this year was badly managed by himself and the UFC.
"These guys don't give a s*** about suspensions. This is some street s***, it's completely different. Khabib wanted to fight him as soon as he could [after the bus attack].
"Did you see how much police was here tonight?
"I don't even know if the city was safe because of all the police here. I think every cop was here. We went above and beyond to make sure this wouldn't be happen. What are you going to do?"
A night of shame for the UFC was compounded when fighting between fans broke out after McGregor's defeat to Nurmagomedov.
Footage has now emerged of vicious fighting outside the arena.
The video, posted on Twitter, shows what appears to be Irish and Russian fans clashing.
One Irish fan is knocked out and left with blood streaming from his face during the disturbing incident.
Khabib Nurmagomedov has defended his UFC lightweight title against Conor McGregor on a night that the fight was overshadowed by a series of disturbing scenes in its immediate aftermath.
After forcing the Irishman into a fourth-round submission, Nurmagomedov scaled the cage and attacked somebody in the octagon-side section.
From the view of this reporter, high in the nosebleed seats, it was impossible to tell who he was after, but the mere sight of it was shocking.
A frenzied atmosphere enveloped the T-Mobile Arena in Las Vegas, as fights broke out in pockets and a huge security presence escorted McGregor away from the octagon.
It is not known if McGregor was in danger, but initial reports suggested that one of Nurmagomedov's team attempted to attack the Irishman.
Nurmagomedov was then escorted - by an even bigger security detail than McGregor received - out of the arena. On his way out, he was pelted with all manner of missiles as the pro-McGregor crowd registered their disgust.
This was a night of shame for the UFC. The incidents were so serious and so disturbing that the Russian fighter was ordered to leave the octagon without celebrating his victory.
In a truly bizarre scene, Bruce Buffer announced the result with neither fighter in the octagon.
Nurmagomedov's victory extends his perfect record to 27-0 and silences many of the doubters who claimed he could never beat a big-name fighter.
The Russian was the better fighter from start to finish. McGregor only showed flashes of his old brilliance that saw him crowned a two-weight UFC champion.
However, analysing the fight seems pointless right now. The bad blood between McGregor and Nurmagomedov has been toxic since April, when the Irishman attacked a bus that was carrying the Russian and a group of UFC fighters.
The events may well be linked back to that night.