Ireland's hugely popular UFC featherweight champion seized Alvarez's lightweight title, knocking the American down three times with punishing left hands in the first round and once again in the next before finishing him off with a brutal four-punch combination at 3min 4sec of the second round.

"Where's my second belt?" McGregor shouted after one was wrapped around his waist, while cheers from a legion of ardent supporters rang around the arena.

The UFC 205 card was the promotion's first foray to New York, which in April became the last US state to legalize professional MMA fights.

The Big Apple proved the perfect stage for McGregor, who has become one of MMA's biggest draws not only for his exploits in the octagon but also for his swagger outside of it.

With his second championship belt delivered and both displayed proudly on his shoulders, McGregor took the microphone and adopted a mock-serious tone.

"I'd like to take this chance to apologize to -- absolutely nobody!" he said, fueling renewed cheers.

Alvarez was in survival mode from the start until referee John McCarthy called a halt as McGregor continued to pepper his prone opponent with punches.

"He's not on my level," said McGregor, who improved to 21-3 in MMA competition and 9-1 in UFC. "You've got to have size, reach, length, you've got to have some attributes.

"If you come here any way equal to me, I'm gonna rip your whole head off."

Alvarez lost the belt he won from Rafael dos Anjos in July and fell to 28-5 (3-2 in UFC).