LOS ANGELES (AFP, REUTERS) - Manny Pacquiao trainer Freddie Roach drew first blood in the war of words with Floyd Mayweather on Wednesday as the countdown to the biggest boxing bout in history got under way.

In remarks to a packed press conference attended by an estimated 600 journalists, Roach said Pacquiao's eagerly awaited May 2 title fight with Mayweather in Las Vegas represented the biggest challenge of his celebrated career as one of boxing's most respected trainers.

But Roach did not mince his words as he wrapped up his comments with a blunt assessment of Pacquiao's task. "We're fighting the best fighter in the world - and we're going to kick his ass," Roach said to cheers from sections of the audience, with Mayweather sitting just a few feet away on the stage in Los Angeles.

"I love challenges and this is the biggest challenge of my life," Roach said. "It's finally here. I've been looking forward to this fight for years.

"It's the toughest fight of our life. I'm going to get my fighter as ready as possible."

Pacquiao, wearing a dark suit and tie, arrived on stage for the news conference as AC/DC's "Thunderstruck" blared through the speakers. He was followed by Mayweather, who was in a light suit with no tie and walked in to Queen's "We Will Rock You."

The two men then stood face-to-face and side-to-side, as a roar of camera shutters tracked their every move.

And while neither boxer took questions from the media packed inside the Nokia Theater in downtown Los Angeles, each exuded confidence.

"This is a fight that the world can't miss... May 2, that's when the world stops." Mayweather said from his podium.

Pacquiao's bout with the unbeaten Mayweather represents the climax to years of will-they won't-they speculation surrounding the possibility of a showdown between the two rivals, widely regarded as the finest fighters of their generation. The fight finally became a reality earlier this year when Pacquiao and Mayweather met informally at an NBA basketball game and expressed a mutual desire to make the contest finally happen.

"It's been a long road, but we're here now," Mayweather said. "It's all about the best fighting the best, and Pacquiao is one of the best."

Pacquiao's camp feel that his age, 36 compared to Mayweather's 38, works to their advantage. "(Mayweather) had a better chance of winning five years ago," said Roach.

"He has slowed down. I think Manny knows how to beat him."

There are no shortage of contentious storylines in this matchup. Roach has exchanged words with Mayweather's father and trainer, Floyd Mayweather Sr., while fight promoter Bob Arum used to work with Mayweather instead of Pacquiao before a falling out split the pair.

May's fight is expected to be the highest-grossing bout in history, with Mayweather reportedly set to collect a staggering US$120 million ($166.6 million) while Pacquiao will pocket a cool US$80 million. Tickets will range from US$1,500 to US$7,500.