Watch Video: Conor McGregor vs Khabib post-fight brawl

Khabib Nurmagomedov thwarted Conor McGregor's bid to regain the UFC lightweight crown Saturday with a dominant submission victory in a grudge match marred by a post-fight brawl.

Nurmagomedov improved to 27-0 in his professional career -- 11-0 in UFC bouts -- with the win at a sold-out T-Mobile Arena in Las Vegas. The UFC lightweight champion had McGregor on the ground less than a minute into the opening round and kept the Irish fighter -- a former UFC featherweight and lightweight champion -- on his back for much of the fight.

McGregor managed to stay on his feet for most of the third round, but the once feared striker couldn't hurt Nurmagomedov and in the fourth, the champion took him down again. McGregor managed to roll away briefly before Nurmagomedov ended up in a full mount, getting him in a brutal rear-naked chokehold that had McGregor tapping out at 3min 3sec of the round. Nurmagomedov had barely released his hold when he launched himself over the Octagon fence, apparently targeting McGregor's training partner Dillon Danis.

As security tried to quell the chaos in the crowd, someone leapt in and took a shot at the exhausted McGregor, who eventually departed in a huddle of security personnel.

Nurmagomedov was also hustled out surrounded by security officers as the pro-McGregor crowd jeered and threw drinks at him. UFC chief Dana White refused to hand Nurmagomedov his belt, saying he feared it would further inflame the crowd. "If I put this belt on you, everyone's going to start throwing shit into the Octagon," he told Nurmagomedov. The ring announcer declared Nurmagomedov the winner after both fighters had departed.

McGregor was returning to mixed martial arts for the first time since November 2016, having detoured in 2017 to a cross-combat boxing bout against Floyd Mayweather which ended in defeat -- but still earned McGregor some $100 million.

His return sparked massive interest, with White predicting the bout would break the UFC record of 1.65 million pay-per-view buys. The crowd included a mass of chanting, flag-waving Irish fans as well as a sprinkling of celebrities including Hollywood stars Matt Damon and Mel Gibson and Northern Ireland golf star Rory McIlroy.

Taking on the powerful champion from Dagestan in his comeback fight was a risk for McGregor, but the drumbeat for a showdown between the two intensified in April, when McGregor was at the center of an extraordinary melee in New York.

McGregor hurled a dolly through a window of a bus filled with other UFC fighters at the Barclays Center -- apparently angered that Nurmagomedov had intimidated one of the Irish fighter's friends, Artem Lobov. Two athletes were injured and McGregor pleaded guilty to disorderly conduct in a Brooklyn criminal court in July in a plea deal that allowed him to avoid jail time. All felony counts were dropped and he was ordered to do five days of community service.