Muhammad Ali, the boxing icon whose battles both inside and outside the ring transcended sport and inspired millions around the world and across generations, has died at 74.

Ali - universally acknowledged as The Greatest, his own claim when he shook the world of heavyweight boxing fighting as Cassius Clay - died late Friday at a hospital in Phoenix, Arizona.

The former world heavyweight champion was admitted Thursday suffering from a respiratory illness, a condition that was complicated by Parkinson's disease, the degenerative condition that Ali had suffered for more than 30 years.

Ali's funeral will take place in his hometown of Louisville, Kentucky, his family said.

"Muhammad Ali was The Greatest. Period. If you just asked him, he'd tell you," US President Barack Obama said among many tributes from around the world.

Obama said he and his wife, Michelle, were "grateful to God for how fortunate we are to have known him, if just for a while; for how fortunate we all are that The Greatest chose to grace our time."

The president later telephoned with Ali's wife, Yolanda "Lonnie" Williams, to personally express the first family's "deepest condolences," a White House spokeswoman said. Obama noted that the "outpouring of love" after the boxer's death was testament to a remarkable life that had changed "the arc of history."

Family spokesman Bob Gunnell said Saturday that Ali died of septic shock due due to natural causes, surrounded in the last hour by family.

A funeral procession through Louisville is planned for Friday, followed by a public memorial service in the city's sports arena, led by a Muslim imam but including other faiths.

"Muhammad Ali was clearly the people's champion, and the celebration will reflect his devotion to people of all races, religions and backgrounds," Gunnell said.

The three-time heavyweight champion was born Cassius Clay on January 17, 1942, in Louisville. He turned professional after winning the 1960 Olympic gold medal in Rome in the light-heavyweight division.

At 22, he knocked out heavyweight champion Sonny Liston in 1964 in a quick and stunning upset. He then joined the Nation of Islam and became Ali, publicly rejecting his "slave name."

Ali became a polarizing figure as a brash advocate for American minorities demanding full civil rights, and as a potent symbol of resistance to the war in Vietnam, when he refused to be conscripted into the US military.

"I ain't got no quarrel with them Viet Cong," he declared. "No Viet Cong ever called me nigger."

He was convicted of draft evasion and stripped of his boxing title, but appealed all the way to the Supreme Court, which overturned the conviction in 1971.

He would not fight again for more than three-and-a-half years, but went on to regain and lose the title twice more.

Ali's most famous fights were the three epic battles he waged against Joe Frazier, and his upset defeat of George Foreman at the relatively advanced age of 32.

Ali used his international fame to spread the appeal of boxing, bringing the third Frazier bout to Manila, Philippines, and to stage the famous Rumble in the Jungle in Kinshasa, Zaire, now the Democratic Republic of Congo.

Ali's career ended with losses to Larry Holmes in 1980 and Trevor Berbick in 1981. Boxing commentators agreed he had fought too long.

After the onset of Parkinson's, Ali 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.

The BBC called Ali the Sports Personality of the Century, and he was one of the most recognized people worldwide.

Former US President Bill Clinton and his wife Hillary offered condolences and described Ali as "courageous in the ring, inspiring to the young [and] compassionate to those in need."

American civil rights campaigner Jesse Jackson said Ali had been willing to sacrifice the championship belt and money for his principles.

Among the sporting tributes, Foreman, who lost his world title to Ali, called his former rival "one of the greatest human beings I have ever met. No doubt he was one of the best people to have lived in this day and age."

Floyd Mayweather, a world champion boxer across five divisions who was the world's highest-paid athlete before retiring last year, said: "There will never be another Muhammad Ali. The black community all around the world, black people all around the world, needed him. He was the voice for us. He's the voice for me to be where I'm at today."

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