Conor McGregor blasted Floyd Mayweather and fight promoters Showtime during the second leg of the pair's media tour ahead of next month's bout.
Mixed martial arts star McGregor has never boxed professionally, but will take on undefeated former five-weight world champion Mayweather in Las Vegas on August 26 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 in Toronto: "Does this mic work? Then f*** that mic."
Addressing Showtime executive vice president Stephen Espinoza, he added: "F*** Showtime too you little weasel - I can see it in your eyes that you're a little b****.
"Cut my mic off? Cut the champ's mic off? Hell no.
"They're trying to throw me off and set me in an uncomfortable position but I thrive in uncomfortable positions.
"I'm 28 years old and I'm getting fight cheques and promoter cheques. When Floyd was 28 he was on Oscar de la Hoya's under card.
"Floyd is an old, weak b****.
"He's never fought a day in his life; he's a runner, he's boxing's biggest b****."
Mayweather was booed throughout by the crowd, who chanted "pay your taxes" at the American in response to reports the 40-year-old still 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 100million 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 media tour continues in New York on Thursday, before the pair appear in London on Friday.