Floyd Mayweather and Conor McGregor's fight that's being billed as the biggest event of the year has found a vocal detractor.

The undefeated boxer and mixed martial artist will step into the boxing ring in Las Vegas today.

Former Olympic champion boxer Oscar De La Hoya has labelled the bout a circus and said boxing may never recover from the gimmick.

McGregor will fight in his first professional boxing bout, while Mayweather is undefeated in 49 fights.

McGregor said he is prepared for 12 three minute rounds of non-stop pace.

"I will go forward and I will put the pressure on him ... I don't see him lasting two rounds. I feel I'll have the decision to end it inside one."

The bout's expected to start around 4pm.