As tributes continue to flow for Muhammad Ali, LeBron James claimed the boxing legend's success inside the ring was secondary to his work outside of it.

The world was left mourning the death of Ali on Friday after the iconic three-time world heavyweight champion passed away aged 74, having been admitted to hospital earlier in the week due to a respiratory illness.

Ali had fought a long and dignified battle with Parkinson's disease for over three decades.

And on a sombre day in the world of sport, four-time NBA MVP James paid tribute to the 'Greatest of All Time', not only for his boxing achievements but for pioneering change as an African-American in the United States.

"So many thoughts come to my mind when I think about the man who passed away yesterday," James said.

"What he represented, as a kid, I gravitated towards him because he was a champion, but I only knew as a kid what he did inside the ring. As I got older and started to be more knowledgeable about sport in general and about the guys who paved the way for guys like myself, I understood that he is the greatest of all time, and he was the greatest of all time because of what he did outside of the ring.

"Obviously we knew how great of a boxer he was, but I think that was only 20 per cent of what made him as great as he was."

James added: "What he stood for - I mean that's a guy who basically had to give up a belt and relish everything he had done because of what he believed in and ended up in jail because of his beliefs.

"That's a guy who stood up for so many different things throughout the times where it was so difficult for African-Americans to even walk in the streets. For an athlete like myself today, without Muhammad Ali, I wouldn't be sitting up here talking in front of you guys. I wouldn't be able to walk in restaurants. I wouldn't be able to go anywhere where blacks weren't allowed back in those days.

"Today I can make trips to China and all over the world and people know my name and face. I give all credit to Muhammad Ali because he was the first icon. He's the greatest of all time and it has nothing to do with his accomplishments in the ring.

"His legacy will obviously live on."

As the Golden State Warriors prepare for game two of the NBA Finals against James' Cleveland Cavaliers, NBA MVP Stephen Curry and coach Steve Kerr also offered passionate tributes.

Curry said "Ali was the example of how you use your platform and speak what you believe no matter what people are saying."

While Kerr continued: "What he did in terms of trying to promote equality, raising the level of consciousness about what was happening in this country - [he was] probably the most influential athlete in the history of our country."