By Ricardo Arduengo

SCOTTSDALE, Ariz. (Reuters) - The death of Muhammad Ali, the former heavyweight champion known as much for his political activism as his boxing brilliance, triggered a worldwide outpouring of affection and admiration for one of the best-known figures of the 20th century.

Ali, who had long suffered from Parkinson's syndrome which impaired his speech and made the once-graceful athlete almost a prisoner in his own body, died on Friday at age 74.

The cause of death was septic shock due to unspecified natural causes, a family spokesman said on Saturday. Ali was admitted to a Phoenix-area hospital, HonorHealth, with a respiratory ailment on Monday.

"He’ll be remembered as a man of the world who spoke his mind and wasn’t afraid to take a chance and went out of his way to be a kind, benevolent individual that really changed the world," the family spokesman, Bob Gunnell, said at a news conference in Scottsdale, Arizona.

Despite Ali's failing health, his youthful proclamation that he was "the greatest" rang true until the end for millions of people around the world who respected him for his courage both inside and outside the ring.

Along with a fearsome reputation as a fighter, Ali spoke out against racism, war and religious intolerance, while projecting an unshakeable confidence that became a model for African-Americans at the height of the civil rights era and beyond.

Stripped of his world boxing crown for refusing to join the U.S. Army and fight in Vietnam, Ali returned in triumph by recapturing the title and starring in some of the sport's most unforgettable bouts.

"I think when you talk about Muhammad Ali, as great an athlete, as great a boxer as he was, he was the greatest boxer of all time, he means so much more to the United States and the world," said Ali's long-time friend, boxing promoter Bob Arum.

"He was a transformative figure in our society."

Bursting onto the boxing scene in the 1960s with a brashness that threatened many whites, Ali would come to be embraced by Americans of all races for his grace, integrity and disarming sense of humor.

"In the end, he went from being reviled to being revered," civil rights leader the Rev. Jesse Jackson told CNN on Saturday.

Pam Dorrough, a tourist in New York's Times Square, admired Ali's refusal to apologize for what he believed.

"The confidence - and I know everybody thought it was an arrogance about him - he always projected a confidence," she said. "And he stood by that."

President Barack Obama, the first African-American to reach the White House, said Ali was "a man who fought for us" and placed him in the pantheon of civil rights leaders Martin Luther King Jr and Nelson Mandela.

"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," Obama said in a statement. "But Ali stood his ground. And his victory helped us get used to the America we recognize today."

Ali's daughter Maryum said on Saturday: "I am happy my father no longer struggles. He is in a better place. God is the greatest."

In New York's Harlem district, fans gathered outside the famous Apollo Theater, where a marquee paying tribute to Ali read: "The greatest of all time. 1942-2016."

Nearby, hundreds more gazed at projections of phrases and images most associated with Ali, such as "float like a butterfly and sting like a bee."

Few could argue with his athletic prowess at his peak in the 1960s, with his dancing feet and quick fists. But Ali became much more than a sportsman. He spoke boldly against racism in the '60s as well as against the Vietnam War.

Ali met scores of world leaders, during and after his championship reign, and for a time he was considered the most recognizable person on earth, known even in remote villages in countries far from the United States.

TRIBUTES POUR IN

Ali's diagnosis of Parkinson's came about three years after he retired from boxing in 1981. Despite his failing health, he appeared at the opening ceremony of the 1996 Atlanta Olympics, stilling the tremors in his hands long enough to light the Olympic cauldron.

From Africa to East Asia to the U.S. South, news of Ali's death brought tributes across the world of sport, entertainment and politics.

In Kinshasa, the city where he battled George Foreman in the 1974 "Rumble in the Jungle" - a city that was then part of Zaire and is now the capital of Democratic Republic of Congo - the fight is remembered as much for its political symbolism as for Ali's tactical brilliance in beating his hulking opponent.

Ali "was an African. He was a Congolese," David Madiawi, a salesman on Kinshasa's Avenue de Commerce, said on Saturday. "He came to Congo to return to the land of his ancestors."

Foreman said Ali was one of the greatest human beings he had met. "No doubt he was one of the best people to have lived in this day and age. To put him as a boxer is an injustice."

Manny Pacquiao, a boxer and politician in the Philippines, where Ali fought Joe Frazier for a third time in a brutal 1975 match dubbed the "Thrilla in Manila," paid homage to Ali's legacy outside the ring.

"We lost a giant today. Boxing benefited from Muhammad Ali's talents but not nearly as much as mankind benefited from his humanity," he said.

Flags were flown at half staff in Louisville, Kentucky, where Ali's modest childhood home on Grand Avenue has been turned into a museum. A funeral will be held in his hometown on Friday.

"LOUISVILLE LIP"

In the brutal world of prize fighting, Ali set himself apart with his wit and fondness for playful verse. "The Louisville Lip," as he was called early in his career, loved to talk - especially about himself.

"Humble people, I've found, don't get very far," he once told a reporter.

But his taunts could be brutal. "Joe Frazier is so ugly that when he cries, the tears turn around and go down the back of his head," he once said about his arch rival. He also dubbed Frazier a "gorilla," but later apologized.

Asked how he wanted to be remembered, Ali said: "As a man who never looked down on those who looked up to him ... who stood up for his beliefs ... who tried to unite all humankind through faith and love.

"And if all that's too much, then I guess I'd settle for being remembered only as a great boxer who became a leader and a champion of his people. And I wouldn't even mind if folks forgot how pretty I was."

Ali was born in Louisville, Kentucky, on Jan. 17, 1942, as Cassius Marcellus Clay Jr, a name shared with a 19th century slavery abolitionist. He changed his name after his conversion to Islam.

Ali is survived by his wife, the former Lonnie Williams, who knew him when she was a child in Louisville, along with his nine children.

(Additional reporting by Brendan O'Brien in Milwaukee, Frank McGurty in New York, Alex Dobuzinskis and Dan Whitcomb in Los Angeles; Writing by Bill Trott and Frank McGurty; Editing by Alison Williams, Leslie Adler and Paul Tait)