Former UN Secretary-General Kofi Annan has eulogised boxing legend Muhammad Ali after his passing over the weekend.

Ali died at the age of 74 at a hospital in the US city of Phoenix, Arizona, after being admitted on Thursday. He died from a respiratory illness, a condition that was complicated by Parkinson's disease.

Paying tribute to the famous boxer, Ghana’s illustrious son said Ali became known as the “greatest” because of his off-the-ring activities which touched several lives.

“Muhammad Ali was known as "the Greatest" not just because of his three World Heavyweight titles, but because he fought some of his fiercest battles outside the boxing ring, on the side of the disenfranchised, the downtrodden and the disadvantaged,” Annan, who served as the seventh Secretary-General of the United Nations from January 1997 to December 2006 said in a Facebook post.

He added: “Muhammad Ali will continue to inspire generations to come to take up the fight against prejudice, injustice and discrimination. This, perhaps, will be his greatest legacy. My thoughts and prayers are with his family and loved ones.”

Ali retired in 1981, having won 56 of his 61 fights. Ali's career ended with one-sided defeats by Larry Holmes in 1980 and Trevor Berbick in 1981, many thinking he should have retired long before.

He fought a total of 61 times as a professional, losing five times and winning 37 bouts by knockout.

Soon after retiring, rumours began to circulate about the state of Ali's health. His speech had become slurred, he shuffled and he was often drowsy.

Parkinson's Syndrome was eventually diagnosed but Ali continued to make public appearances, receiving warm welcomes wherever he travelled.

He lit the Olympic cauldron at the 1996 Games in Atlanta and carried the Olympic flag at the opening ceremony for the 2012 Games in London.