Featherweight champion Conor McGregor capped the UFC's triumphant return to New York by knocking out Eddie Alvarez to claim the lightweight crown.

McGregor become the first fighter in the history of the Ultimate Fighting Championship to hold two UFC belts simultaneously.

The southpaw downed Alvarez three times in the opening round, leaving the champion stunned and on the back foot for the entire five minutes. And it didn't get any better for the American from Philadelphia.

The cocky Irishman played to the crowd of 20,427 at the start of the fight, running a circle around the mat - a victory lap of sorts - before he even threw a punch. He dubbed himself this week the "King of New York".

Alvarez's attempts to take the fight to the mat were thwarted by the 28-year-old McGregor, who continued to dictate the pace in the second round in front of a packed house at Madison Square Garden, most of whom were cheering for the Irishman.

McGregor put his hands behind his back at one point, taunting and toying Alvarez to hit him.

Midway through the second round, McGregor unleashed another devastating combination. This time there was no way back for Alvarez as he pounced to finish the fight.

McGregor once again proved his value to the UFC by putting on a spectacular show with another superb knockout win to go with his 13-second defeat of Jose Aldo for the featherweight crown.

"Eddie is a warrior - but he shouldn't be in here with me. I'm at a different level. You've got to have size, reach, length, you've got to have attributes," he told ring announcer Joe Rogan following the bout.

Critisised by some for his brash attitude and verbal hostility towards opponents, McGregor was in no mood to make amends. 

"I'd like to take this chance to apologise ... to absolutely nobody," he roared before taking his two belts and scaling the side of the octagon to celebrate.

The 32-year-old Alvarez had won the title from Rafael Dos Anjos back in July. After McGregor was done avenging his only UFC defeat to Nate Diaz, he immediately set his sights on taking that second belt.

With the ban on mixed martial arts in New York state lifted in March of this year, the UFC returned after an absence of more than two decades, staging a card stacked with fights and backed up by a tonne of hype.

Conor McGregor, left, fights Eddie Alvarez during a lightweight title mixed martial arts bout at UFC 205 at Madison Square Garden in New York [Julio Cortez/AP]

Source: Agencies