McGregor donned a slick black suit, which appeared to be pinstriped. Upon closer inspection, you could read hundreds of expletives in the lines. Mayweather sported an American stars and stripes TMT track suit.
The world tour has four stops, wrapping up Friday in London. The fighters head to Toronto, Canada on Wednesday and will meet Thursday in New York.
"I'm absolutely honored to be here before you," McGregor said at the beginning of Tuesday's news conference. "Give you this great spectacle ... somebody through has got to go. He's in an [expletive] track suit, he cant even afford a suit anymore. He is [expletive]. There is no other way about it. His little legs. His little car. His little head. I'm going to knock him out inside four rounds. Mark my words."
McGregor (19-3 MMA) has been training hard for his boxing match against the 49-0 un-retired Mayweather.
But Mayweather has also never been short on words.
"Money" lobbed some insults back at "The Notorious."
"He look good for a seven-figure fighter, he look good for a seven-figure fighter," Mayweather said. "I look good for a nine-figure fighter."
"I ain't back down from no [expletive] body. You line 'em up, and I knock 'em down like bowling pins," Mayweather said. "And August 26th, Imma knock this [expletive] out too!!"
"Let me show you [expletives] what a $100 million fighter looks like. Still got 100 million, and ain't even ever touch this [expletive]."
Mayweather called himself an "old man" and said he wasn't "The same fighter" he was 10 years ago, five years ago or two years ago. He said he still has "enough" to beat McGregor.
"And we know Mr. Tapout like to quit," he said. Mayweather added that McGregor will quit either on his face or on his back.
McGregor's microphone was cut off during the exchange. Both fighters eventually met face-to-face at center stage with UFC president Dana White stepping in between. The stare-down lasted for about 90 seconds.
The attendance for the news conference was about 11,000. Mayweather vs. McGregor is planned for Aug. 26 at T-Mobile Arena in Las Vegas. Showtime will air the pay-per-view bout for about $90. For another $10, you can watch it in high-definition.
"It went good," White said after the presser. "Listen, there are some glitches we need to fix. Like we couldn't hear anything Conor or Floyd were saying when they were on the mic because the audio was really bad up there. The crowd was awesome. The energy in here was insane. The media was great. Again, not our typical style of press conference but it was fun."
White met up with McGregor before the fight at a Los Angeles-area airport. He'll be on the tour at each stop.
The boxing bout will be fought at 154-pounds. All free tickets for the tour were sold out. McGregor is expected to pocket about $100 million from the fight, while Mayweather should cash in for about $300 million.
According to a poll on the Showtime Boxing Twitter account, 54 percent of the voters chose McGregor as the winner of Day 1 of the media tour.