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Floyd Mayweather (left) and Conor McGregor (right) have toured North America to promote their fight. Getty

The upcoming boxing fight between undefeated Floyd Mayweather and UFC star Conor McGregor continues to draw heavy interest from bettors. Mayweather, considered one of the greatest pound-for-pound boxers of all-time, will be facing McGregor, who has never been in a professional fight, on Saturday night in Las Vegas.

Most bettors have gravitated to the underdog in the 154-pound bout.

According to online betting site OddsShark, Mayweather entered at -2,250, while McGregor was listed as a +950 underdog. But Mayweather is now listed at -400 on OddsShark, while McGregor has been given +300 odds.

“Many sportsbooks are seeing something like 10 times the number of bets on McGregor than they are on Mayweather,” Jon Campbell, an analyst at OddsShark, said in a press release. “The media hype has been a total circus and McGregor just keeps drawing bets. I’ve never seen anything like this kind of line movement for a sporting event.”

Campbell believes that casual bettors have driven up the odds for McGregor. “It’s hard to fathom but this line could come down even more as the fight gets closer and more cash flows in on McGregor,” he said.

The betting odds have fluctuated, though. On Aug. 14 OddsShark listed McGregor as a +375 underdog, while Mayweather was a -500 favorite. On June 18, McGregor’s odds were listed at +500 with Mayweather listed as a -700 favorite, according to Westgate Las Vegas Super Book.

Like OddsShark, Bovada.lv also currently lists Mayweather at -400 and McGregor at +300. VegasInsider.com has Mayweather at -500 and McGregor at +400.

One possible reason for McGregor's boost in odds is the Nevada State Athletic Commission's decision last week to allow the fighters to use 8-ounce boxing gloves instead of the standard 10-ounce gloves for fights above 147 pounds. Lighter gloves should help McGregor, who typically wears 4-ounce gloves in UFC fights.

McGregor has also boasted of knocking down veteran boxer Paulie Malignaggi in a sparring session.

But odds may start favoring Mayweather in the final days. Against Andre Berto, who had 30-3 record ahead of their September 2016 fight, Mayweather had a -3,000 odds. Mayweather was also a -200 favorite over Manny Pacquiao.