Muhammad Ali, who declared “I am the greatest’’ and proved it many times over, infuriating some and captivating countless more as he floated like a butterfly and stung like a bee on his way to winning the world heavyweight championship a record three times, becoming perhaps the most widely recognized person on the planet, died Friday in Phoenix. He was 74.

Mr. Ali had long suffered from Parkinson’s syndrome. The condition was understood to be a consequence of his boxing career. He was hospitalized in Phoenix with respiratory problems earlier this week, and his relatives gathered around him. His family said he died of septic shock.

The news let loose a flood of tributes.

‘‘Ali will never die,’’ Don King, who promoted some of Mr. Ali’s biggest fights, told the Associated Press. ‘‘Like Martin Luther King his spirit will live on, he stood for the world.’’

‘‘That he had come full circle from being reviled to being revered, from being dismissed to being embraced, I think he found a certain joy in that,’’ said the Rev. Jesse Jackson, the civil rights leader and a longtime friend.

President Obama, who keeps a pair of boxing gloves worn by Mr. Ali in his private study off the Oval Office, said in a 564-word statement that the boxer ‘‘shook up the world and the world is better for it.’’

A fame that transcended the ring“There’s not a man alive who can whup me,’’ Mr. Ali declared before his first bout with Joe Frazier, “the Fight of the Century’’ in 1971. “I should be a postage stamp. That’s the only way I’ll ever get licked.’’

In fact, Mr. Ali wasn’t invincible. He lost that fight, as well as four later prizefights. But he finished with a career record of 56-5, 37 of those victories by knockout.

“Ali! Ali! Ali!’’ became one of the 20th century’s most frequently heard chants, an acclamation of tribute from a tribe whose members encompassed much of humankind.

When Nelson Mandela was inaugurated as president of South Africa, he corrected a guest who said that he and Mr. Ali were the world’s two most beloved and unifying figures. “If I was in a crowded room with Ali,’’ Mandela said, “I would stop what I was doing and go up to him. He is the Greatest.’’

Mr. Ali’s star power extended to the world of diplomacy. Jimmy Carter appointed him special envoy to lobby African leaders to support the Olympic boycott in 1980. Mr. Ali helped obtain the release of 14 US hostages in Iraq in 1990. Ten years later, he was named a United Nations ambassador of peace.

In neither duration nor significance could any of Mr. Ali’s boxing matches rival his struggle with the Selective Service System. “When they draft me, I won’t go,’’ Mr. Ali had declared of the Vietnam War. “I ain’t got no quarrel with them Viet Cong. It ain’t right. They never called me nigger.’’

Having refused induction in 1967 as a conscientious objector, Mr. Ali was sentenced to five years in prison and a $10,000 fine. He appealed the ruling, and his conviction was unanimously overturned by the Supreme Court, in 1971. But Mr. Ali, who had had his championship and boxing license taken away, lost three and a half years of his athletic prime.

What it gained Mr. Ali was a status and personal authority that extended far beyond the realm of sports. His political stance offended many, but to others it made him a hero and martyr. A 1968 Esquire magazine cover famously showed Mr. Ali with arrows sticking out of him, like St. Sebastian.

A ’60s catch phrase held that the personal was the political. In Mr. Ali’s fists the pugilistic was political. He once described his style as “Be loud, be pretty, and keep their black-hatin’ asses in their chairs.’’ His very name excited controversy.

For years, the decision whether to use “Muhammad Ali’’ or “Cassius Clay,’’ the name he rejected in 1964 when he joined the Nation of Islam, was a clear-cut political statement. The New York Times Index didn’t stop referring to him as Cassius Clay until 1972. “Cassius Clay is a slave name,’’ Mr. Ali declared. “I didn’t choose it, and I didn’t want it.’’

When Sports Illustrated named Mr. Ali Sportsman of the Century, in 1999, it didn’t so much make a selection as ratify a foregone conclusion. His two title fights with Sonny Liston, his three bouts with Frazier (two of them title fights), his title fight with George Foreman — “the Rumble in the Jungle,’’ Mr. Ali dubbed it — rank among the most celebrated contests in boxing history, epic clashes that transfixed much of the world.

Yet to describe Mr. Ali as a prizefighter would be akin to describing Christopher Columbus as a sailor. Accurate so far as it goes, such a designation leaves out the enormous symbolic weight each man carried — not to mention his navigation of a new world.

Actually, Mr. Ali did Columbus one better: He had multiple new worlds to navigate — and also conquer: media, celebrity, race, politics, the ’60s. Few of those worlds were altogether new, of course. Several had bedeviled his fellow African-American and predecessor as heavyweight champion, Joe Louis. But Mr. Ali made everything around him, whether new or old, seem his personal possession.

“I am the astronaut of boxing,’’ he proclaimed. “Joe Louis and [Jack] Dempsey were just jet pilots. I’m in a world of my own.’’

In that world, Mr. Ali reduced the media to the status of personal PR staff and turned race into a kind of game, at once comical and deadly serious. He hosted his own celebrity as if it were a party, one to which he invited the entire human race. Bringing politics into the boxing ring, he all but single-handedly tore down the barrier between sports and the real world.

As for the ’60s, he was one of its key figures, both symbol and instigator, like the Beatles (whom he clowned with in Miami before the first Liston bout) or Mao Zedong. Mao had his Red Guards.

Mr. Ali, a one-man great cultural revolution, required only himself. His equivalent to “Quotations from Chairman Mao’’ was a seemingly inexhaustible stream of boasts, jeers, and rhymes delivered in his silken, whispery rasp. Mr. Ali might even qualify as the first rapper. In 1964, Columbia Records released an LP of him reciting his verse, “I Am the Greatest.’’

As a poet, Mr. Ali was often prescient (“You think the world was shocked when Nixon resigned?/Wait till I whup George Foreman’s behind’’). He was never modest (“They’ll all fall in the round I call!’’). He gave good advice (“If you want to lose your money/Bet on Sonny’’) and wise counsel (“I got speed and endurance/You’d better increase your insurance’’).

Above all, he was witty (“I done wrassled with an alligator,/I done tussled with a whale,/Handcuffed lightnin’, threw thunder in jail./Only last week I murdered a rock,/Injured a stone, hospitalized a brick,/I’m so mean I make medicine sick’’). His love of rhyme had a counterpart in a fondness for magic tricks and practical jokes.

A master of many media

How great was Mr. Ali’s charisma? He played himself in the movie of his life — called, inevitably, “The Greatest’’ (1977). A quarter century later, Will Smith received a Best Actor Oscar nomination for the title role in “Ali.’’ Mr. Ali even starred in a short-lived 1969 Broadway musical, “Buck White.’’

There was a childlike quality to Mr. Ali. He was the hero of an animated children’s television series in the 1970s and bested Superman in a 1978 DC comic book. That quality was central to the universality of his appeal. He had the simplicity and cunning, not to mention mischievousness, of a tall-tale folk hero.

In his way, Mr. Ali was as much of an American original — and as larger than life — as a Paul Bunyan. Mr. Ali was king and clown, warrior and bard, all rolled into one. He would turn a press conference or weigh-in into performance art of the most outrageous sort.

“Frazier! I want Frazier! Joe Frazier!,’’ he brayed and hollered in pursuit of his most formidable rival. It was a rallying cry, a schoolyard taunt, a jubilant put-on. “If my fans think I can do everything I say I can do,’’ Mr. Ali once confessed, “then they’re crazier than I am.’’ An impish sense of humor and keen self-awareness almost always kept him just short of going too far.

Almost always: Mr. Ali’s verbal abuse of former heavyweight champion Floyd Patterson before a 1965 bout turned vicious, as did his relentless mocking of Frazier, whom he unfairly painted as an Uncle Tom.

Unlike Frazier or Liston, Mr. Ali was nothing like the classic plug-ugly prizefighter. “Look at this face,’’ he said, “I’m the prettiest thing that ever lived.’’ It was only a slight exaggeration. With his stunning combination of strength, beauty, and wit, Mr. Ali could seem like a demigod.

That didn’t make him a saint. His prodigious womanizing led his doctor, Ferdie Pacheco, to call Mr. Ali a “pelvic missionary.’’ Mr. Ali cast his lot with a group that espoused a particularly noxious vision of race relations when he joined the Nation of Islam, commonly known as the Black Muslims. “When Cassius joined the Black Muslims,’’ lamented Martin Luther King Jr., “he became a champion of racial segregation.’’

Mr. Ali would eventually become a Sunni Muslim and profess a message of universal brotherhood — but not before turning on Malcolm X, who had recruited him, when Malcolm broke with the Black Muslims.

Standing 6 foot 3 inches, Mr. Ali weighed around 190 pounds when he first won the title. His fighting weight eventually rose to 220 pounds. Perhaps Mr. Ali’s key physical attribute was an 80-inch reach, which allowed him to evade punches as he relentlessly jabbed at an opponent.

Where other fighters would duck or catch punches, Mr. Ali would lean back from them. He held his hands by his side, rather than up high to protect his head. His phenomenal speed and agility allowed him to fight like a middleweight (Sugar Ray Robinson was Mr. Ali’s idol) with a heavyweight’s power.

In his trademark white trunks and red-tasseled shoes, he prowled the ring with a dancer’s grace — the New York City Ballet’s George Balanchine marveled at the speed and dexterity of his legwork — showing off with his Ali Shuffle. “I was the Elvis of boxing,’’ he once said. Even so, Mr. Ali was as much fighter as boxer.

He had a remarkable ability to take a punch, as he demonstrated in his three fights with Frazier — during the third, the Thrilla in Manila, above all. And despite suffering a broken jaw, Mr. Ali went the distance in his 1973 loss to Ken Norton.

Cassius Marcellus Clay Jr. was born in Louisville on Jan. 17, 1942. His father was a sign painter. His mother, Odessa (Grady) Clay, was a domestic. When Mr. Ali was 12, his bicycle was stolen. The police officer who investigated the theft was a boxing trainer. He took on Mr. Ali, who would win 100 out of 108 amateur fights, taking six Kentucky Golden Gloves titles, two national Golden Gloves titles, and two national Amateur Athletic Union titles.

Mr. Ali won an Olympic gold medal in the light-heavyweight division at the 1960 Games, in Rome. Debuting as a professional on Oct. 29, 1960, he won 19 consecutive bouts — 15 by knockout.

As early as 1962, Mr. Ali proclaimed, “I’m not the greatest; I’m the double greatest. Not only do I knock ’em out, I pick the round.’’ Dubbed the “Louisville Lip,’’ he began to alienate fight fans and promoters.

That year Mr. Ali told an English newspaper, “At home I am a nice guy, but I don’t want the world to know. Humble people, I’ve found, don’t get very far.’’

Two stunning events in nine daysMr. Ali was widely expected to lose when he fought Liston in Miami Beach on Feb. 25, 1964. Only 7 percent of sportswriters in attendance picked him to wrest the title from Liston, a brutal presence in the ring who refused to take his challenger seriously. Mr. Ali wore him down with his speed. Claiming he’d dislocated his shoulder, Liston refused to come out when the bell rang for the seventh round.

The shock over the upset was great. It paled, though, before the response nine days later when he announced he’d joined the Nation of Islam. Mr. Ali had taken his talent for effrontery to a new, much more serious level.

An Ali-Liston rematch was to be held at Boston Garden on Nov. 13, but Mr. Ali was diagnosed with a hernia. Postponed? until May 25, 1965, the fight was moved to Lewiston, Maine. Liston was a 9-5 favorite, but Mr. Ali knocked him out less than two minutes into the first round.

Fight fans continue to argue over the “phantom’’ punch that brought down Liston, with not a few contending he threw the fight. Sports Illustrated’s Neil Leifer, among others, took a photograph seconds after Liston went down. It has become one of the totemic images in 20th-century sports: Mr. Ali roars in sheer animal triumph, terrifying yet astonishingly beautiful, exulting over the prostrate form of the fearsome man he has felled.

Over the next two years, Mr. Ali defended his title eight times. Then his refusing induction and subsequent conviction kept him from the ring for more than three years. Mr. Ali regained his boxing license in 1970. He fought Frazier, the defending champion, on March 8, 1971.

Even Mr. Ali had to struggle to come up with words sufficiently hyperbolic to describe the event. “Not since time began will there be a night like this,’’ he proclaimed. Frazier took a 15-round decision. Three years later, Mr. Ali outpointed Frazier in New York, winning the right to meet Foreman in Kinshasa, Zaire (now the Democratic Republic of Congo).

Foreman, who had taken the title from Frazier in 1973, was an overwhelming puncher, and Mr. Ali was a 7-1 underdog. Rather than avoiding Foreman’s blows, he covered up and took the punishment. It was his most daring strategy, the Rope-a-Dope. Flailing away, Foreman exhausted himself. Mr. Ali knocked him out in the eighth round. Mr. Ali had the title back.

To retain it he had to face Frazier one more time. Oct. 1, 1975, was the Thrilla in Manila. It was the best heavyweight fight I have ever seen,’’ Pacheco said, “and the closest to death that both fighters ever came.’’ Frazier’s trainer refused to let him back in the ring after 14 rounds. Mr. Ali never again spoke ill of Frazier. “That’s one hell of a man,’’ he said. “God bless him.’’

A struggle to end careerUrged to quit the ring to protect his health, Mr. Ali refused. He fought a succession of undistinguished opponents, in locales ranging from Kuala Lumpur to Landover, Md. The one constant was Mr. Ali’s increasing difficulty in winning. Age finally caught up with him on Feb. 15, 1978, when the unheralded Leon Spinks took a split decision. Mr. Ali reclaimed his title seven months later in a rematch with Spinks. “I’d be a fool to fight again,’’ he said in 1979, announcing his retirement.

Unable to resist the lure of the spotlight, he returned to the ring on Oct. 2, 1980, losing badly to his successor as champion, Larry Holmes. There was one more fight, another one-sided defeat, against Trevor Berbick, in December of 1981. In 1984, it was announced that Mr. Ali had been diagnosed with Parkinsonism.

Slowed by illness, Mr. Ali kept an increasingly low profile. The eyes of the world were once again on him, though, when he raised a trembling arm to light the Olympic flame at the 1996 Games, in Atlanta.

Mr. Ali was awarded the Presidential Medal of Freedom, the nation’s highest civilian honor, in 2005.

In addition to his wife, Yolanda and son, Assaad, Mr. Ali leaves a brother, Rahaman; three daughters from his second marriage, to Belinda Boyd, Maryum, Rasheeda, and Jamilla, and a son, Muhammad Jr.; two daughters from his third marriage, to Veronica Porsche, Hana, and Laila; and two daughters from other relationships, Miya and Khalilah.

A family spokesman said his funeral will be held at 2 p.m. Friday at the KFC Yum! Center in Louisville. It will be open to the public, according to the AP. Eulogies will be given by former president Bill Clinton, Billy Crystal, and Bryant Gumbel.

Mark Feeney can be reached at mfeeney@globe.com.