McGregor's foreboding final speech ahead of mega-fight

UFC champion Conor McGregor has sent down his final verbal strikes, declaring the boxing world should have 'left him where he was'.

As McGregor and Mayweather faced off for the final time ahead of their money-spinning boxing duel that could become the richest fight in history, the Irishman pulled no punches.

“We are prepared for 12 three-minute rounds of non-stop pace. I will go forward, I will put the pressure on him and break this old man, trust me on that, we are more than ready," McGregor said.

"Eight-ounce gloves, he made a big, big error in my opinion. I don’t see him lasting two rounds. I feel I will have the decision to end it inside one [round].

"I’m gonna f*** this boy up, make no mistake. And when it’s all said and done, you know what, I’m gonna feel a little bit sad.

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"Because you all shoulda kept your mouths shut, you shoulda left me over in that other game that I’m from, that more ruthless game where we bounce heads off canvas and drill them into the floor.

"You shoulda left me where I was, because this man is not on my level, he’s not even a quarter of the man I am. So we’re gonna see and you are all going to eat your words in there."

Mayweather on the other hand, who bombarded McGregor with expletives and homophobic slurs during last month's frenzied publicity blitz, did not swear once during Wednesday's news conference.

The 40-year-old looked relaxed throughout, even taking time to quietly admonish members of his entourage shouting at McGregor.

And rather than the abuse of last month, Mayweather praised McGregor as a "helluva fighter, a stand-up guy and a tough competitor."

"It's not going to be an easy fight," Mayweather said.

"It's going to be blood sweat and tears. "He's the best at what he does, I'm the best at what I do. It comes down to the two competitors going out there and displaying our skills."

But Mayweather's words came with a warning for McGregor.

"After 21 years I've been hit with everything and I'm still right here," Mayweather said.

"And if you give it, you must be able to take it.

"Anything and everything in boxing that can be done, I've done it. I was born a fighter, I will die a fighter. He's going to bring his best. But it's not going to be easy, Conor."

McGregor is hoping his punching power can catch Mayweather out on Saturday. But the American veteran warned he had faced plenty of hard hitters before.

"Manny Pacquiao got bombs, Canelo (Alvarez) got bombs, Shane Mosley got bombs. But remember this -- I got a great chin. And the same way you give it, you've got to be able to take it."

McGregor, the massive underdog for this weekend's bout, also verbally abused a heckler in the audience who predicted he would be knocked out.

McGregor faces his first ever professional boxing contest against Mayweather, the undefeated former welterweight king who has come out of a two-year retirement to take on the Irish mixed martial arts star.

However McGregor, the 29-year-old from Dublin, insisted he was ready to stun the world of combat sports by upsetting Mayweather, who would improve his record to a perfect 50-0 with a win.

The showdown between Mayweather and McGregor was confirmed in June after a protracted guessing game about whether the two fighters from different sports would meet.

Saturday's bout could become the most lucrative fight in history according to Showtime Sports television executive Stephen Espinoza.

Mayweather could add another $200 million to his career earnings depending on pay-per-view sales while McGregor, who only four years ago was a struggling former plumber's apprentice living on welfare, could collect $100 million.

This will be the most widely distributed pay per view in history," Espinoza said.

"That's not hyperbole that's fact. We are well on the way to a record-setting event."