American Dustin Poirier defeated Conor McGregor on Saturday in Las Vegas, winning their lightweight UFC trilogy fight by a TKO after the Irishman broke his ankle in the ending moments of the opening round.

The fight was officially ruled as a "doctor's stoppage" at the end of the first round after a McGregor swung and missed a punch then stumbled backward only to break his ankle in a horrific fashion at the edge of the octagon.

As the Irish fell down on the ground, Poirier pounced on him and rained on him a series of punches and elbows. Mc Gregor was saved by the bell at the T-Mobile Arena.

The lightweight fight was attended by former US president Donald Trump. As he entered the arena, he was welcomed with loud boos.

The other celebrities who attended the event were Hollywood actor Jared Leto and DJ Steve Aoki.

According to UFC President Dana White, the gate revenue stands at around $16.5 million.

As far as the match is concerned, officials said the freak injury that McGregor sustained was to his lower tibia and that he was expected to undergo surgery on Sunday to repair the damage.

The scheduled five-round rubber match was expected to settle the score and complete the trilogy between the former two-weight UFC champ McGregor and his longtime rival Poirier.

In the run-up to the third fight, the two fighters traded barbs. McGregor insulted Poirier's wife in a tweet and told Poirier he was going to leave the ring in the box. Instead, it was the opposite.

"He fractured it on one of the checks at the beginning, then broke it with the punch. I felt something. He was kicking me hard. It was probably cracked then on the twist and punch [it broke]," Poirier said.

Poirier said his payback was karma. "This guy is a dirt bag," Poirier said. "Sometimes these things happen."

In an expletive-laden interview inside the octagon as he waited for the stretcher to take him out, McGregor said: "I was boxing his bleeding head off. Kicking his bleeding leg off. This is not over," said McGregor, who has had just one win in four and a half years.

In 2014, McGregor knocked out Poirier in under two minutes and Poirier returned the favour in January, demolishing McGregor with a second-round KO.