President Barack Obama has paid tribute to legendary boxer and activist Muhammad Ali, who has passed away at the age of 74.

The three-time heavyweight world champion, who is simply known as 'The Greatest', died at a hospital in Phoenix, Arizona in the United States following respiratory problems.

As well as winning 56 of his 61 fights, the Louisville-born fighter, who converted to Sunni Islam in 1975, was known for his activism and as a result of refusing to fight in the Vietnam war, he was stripped of his heavyweight title.

Following the news of his passing, Obama released a statement saying that he came to know Ali "not just as skilled a poet on the mic as he was a fighter in the ring, but a man who fought for what was right. A man who fought for us. He stood with King and Mandela; stood up when it was hard; spoke out when others wouldn't.

"His fight outside the ring would cost him his title and his public standing. It would earn him enemies on the left and the right, make him reviled, and nearly send him to jail. But Ali stood his ground. And his victory helped us get used to the America we recognize today.

"Muhammad Ali shook up the world, and the world is better for it. We are all better for it. Michelle and I send our deepest condolences to his family, and we pray that the greatest fighter of them all finally rests in peace."

Ali's family have confirmed that he will be laid to rest in a funeral in Louisville next week.