'I'm going to knock him out inside four rounds'

The first press conference of Conor McGregor and Floyd Mayweather promised fireworks and it didn't disappoint as both men traded brutal barbs.

The Irishman and the American came face-to-face for the first of a series of worldwide media engagements since announcing their August 27 fight.

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McGregor took to the stage with his trademark bravado, vowing to knock out Mayweather inside four rounds during their Las Vegas showdown.

The UFC champion will make his professional boxing debut against the 49-0 undefeated former world champion at the T-Mobile Arena.

The pair have traded barbs before and since the fight was announced, but both were in particularly spiky form as the world tour to promote their bout got under way.

McGregor took to the stage first, taking shots at Mayweather's reported recent request to be allowed an extension on his 2015 tax bill in a typically expletive-laden performance.

But 'The Notorious' drew the biggest cheer from a fired-up crowd in the Staples Center when issuing his big-fight prediction.

In a crisp three-piece suit, McGregor began with a sartorial blow: "He's in a f*****g tracksuit! He can't even afford a suit anymore.

"He is f****d. There's no other way about it. His little legs, his little core, his little head... I'm going to knock him out inside four rounds mark my words.

"As far as the fight, he will be unconscious inside four rounds. My movement, my power, my ferociousness - he's not experienced it."

"This is a limited set of rules, that makes this half a fight, quarter of a fight, it's not a true fight.

"If it was a true fight, it wouldn't take one round."

Mayweather's efforts did not quite hit the same notes as McGregor's wide-eyed monologue and the Irishman even managed to steal the attention when the boxer was speaking.

As Mayweather claimed: "I don't give a f*** if it's a ring, if it's an octagon, put me in there and I will kick ass," McGregor countered with; "Don't talk s***, you do give a f*** if it's an octagon."

As laughter rolled down from the 20,000 in attendance, the UFC lightweight champion again raised a cheer when cutting off his opponent to claim "You haven't knocked anyone out in 20 years" when the 40-year-old predicted a win inside the distance of his own.

Perhaps bruised, Mayweather retreated to a topic he knows best - money.

"He looks good for a seven-figure fighter, an eight-figure fighter," Mayweather said.

"But motherf***** I'm a nine-figure fighter.

"This b**** made three million dollars his last fight. We know that's training camp money for me."

Mayweather and McGregor will take part in four press conferences, over four days, in four major cities: Los Angeles, Toronto, New York and London.