Boxing legend Muhammad Ali, the three-time world heavyweight champion known simply as "The Greatest," passed away Friday. He was 74.

According to The Associated Press, Ali had been hospitalized in Phoenix earlier this week with respiratory problems, which were complicated by the Parkinson's disease he was diagnosed with in the 1980s. Family members flew from around the country in late Thursday and early Friday to be by his side.

Flags were ordered flown at half-staff in Ali's hometown of Louisville, Kentucky, which will hold a memorial service today.

"The values of hard work, conviction and compassion that Muhammad Ali developed while growing up in Louisville helped him become a global icon," Louisville Mayor Greg Fischer told The AP. "As a boxer, he became The Greatest, though his most lasting victories happened outside the ring."

A political activist, author and towering figure in popular culture, the silver-tongued Ali was frequently as controversial outside of the ring as he was inside of it.

Born Jan. 17, 1942 as Cassius Marcellus Clay Jr., and originally boxing under the name Cassius Clay, he gained international fame in 1964 when he won the world heavyweight championship in a stunning upset over Sonny Liston. Boasting a career record of 56-5, with 37 knockouts, Ali's fights -- to say nothing of his taunts -- are legendary.

Proclaiming himself "the greatest," a title few could argue with, Ali spouted now-iconic phrases like “Float like a butterfly, sting like a bee” and “Your hands can’t hit what your eyes can’t see."

The first fighter to win the world heavyweight title three times, Ali was banned from boxing for more than three years in the late 1960s after being found guilty on draft-evasion charges (a conscientious objector, his conviction was later overturned by the U.S. Supreme Court). He returned to reclaim the belt in 1974 in the famed "Rumble in the Jungle" against George Foreman, a fight that's been called the greatest sporting event of the 20th century.

Ali's long reach extended beyond boxing and into politics, television and, yes, comic books, where in 1978 he famously went toe to toe with the Man of Steel in DC Comics' "Superman vs. Muhammad Ali," an oversize book by Neal Adams and Denny O'Neill (the two heavyweights eventually team up to prevent an alien invasion of Earth). More recently, the champ appeared in Darwyn Cook's "DC: The New Frontier," where as a young Cassius Clay he loses to Ted "Wildcat" Grant, and again in "Before Watchmen: Comedian," which briefly revisited his first defeat of Liston.

He also starred in his own short-lived 1977 animated series, appropriately titled: "I Am the Greatest: The Adventures of Muhammad Ali."

Ali retired from boxing in 1979, only to return the following year to face Larry Holmes in a fight that was stopped in the 11th round by Ali's trainer. He fought once more in 1981, against Trevor Berbick, but lost that match too.

Three years later, Ali was diagnosed with Parkinson's, an apparent result of repeated blows to the head during his three decades in the ring. The degenerative disorder eventually robbed Ali of his speech.

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