Conor McGregor blasted Floyd Mayweather and fight promoters Showtime during the second leg of the pair's media tour in Toronto.

UFC star McGregor has never boxed professionally, but will take on undefeated former five-weight world champion Mayweather in Las Vegas on August 26 at the T-Mobile Arena under normal boxing rules.

The pair are promoting the fight across four cities in as many days, but after McGregor's microphone appeared to be cut as Mayweather spoke in Los Angeles on Tuesday, the Irishman declared to a raucous crowd of 16,000: "Does this mic work? Then f*** that mic."

Mayweather was booed throughout by the crowd, who chanted "pay your taxes" at the American in response to reports the 40-year-old owes the Internal Revenue Service millions of dollars from his 2015 fight against Manny Pacquiao.

Responding to a McGregor quip that he cannot read, Mayweather - who had produced a cheque 100 million US dollars on Tuesday - said: "They said I'm the mother f***** that can't read: I do numbers, I make money."

McGregor responded: "You owe money".

Mayweather, who then took an Irish flag from the crowd and draped himself in it, continued to direct insults at McGregor before the pair were separated.

The undefeated fighter, who retired in 2015 with a record of 49-0, said McGregor was a very dangerous opponent - despite most media labeling the fight as a serious mismatch.

He told reporters that McGregor lost three times by submission, but has never been defeated on his feet.

"The last time I checked he's still a professional fighter," Mayweather said of his lightly-regarded opponent. "He's a stand-up fighter. He never took a loss fighting straight up. I can't overlook that, because one shot can end your whole career. He's a tough competitor but it is what it is."

The media tour continues in New York on Thursday, before the pair appear in London on Friday.