Boxing legend Muhammad Ali dead at age 74

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Muhammad Ali, the legendary boxer who proclaimed himself "The Greatest" and was among the most famous and beloved athletes on the planet, died Friday in Phoenix, Arizona, a family spokesman said.

Ali, 74, had been at a Phoenix hospital since Thursday with what spokesman Bob Gunnell had described as a respiratory issue.

The three-time heavyweight champion also had battled Parkinson's disease since 1984.

Laila Ali "truly appreciates the outpouring of love for her family, as she spends quality time with her dad," a statement issued by her business manager said.

The boxing legend also was hospitalized in January 2015 with a urinary tract infection. He was hospitalized in December 2014 with pneumonia.

Laila Ali posted a throwback photo of her famous father with her daughter, Sydney, who was born in 2011.

"I love this photo of my father and my daughter Sydney when she was a baby! Thank for all the love and well wishes. I feel your love and appreciate it!!" Laila Ali, herself a former world champion boxer, wrote.

Former six-time world champion Sugar Ray Leonard tweeted: "Prayers & blessings to my idol, my friend, & without question, the Greatest of All Time @MuhammadAli ! #GOAT

Boxing great Manny Pacquiao asked for his many fans to "Please keep @MuhammadAli in your thoughts and prayers. With God, all things are possible."

Actor Liev Schreiber, the voice of many documentaries including "Ali-Frazier I: One Nation... Divisible," wrote online, "Thinking about the greatest tonight(.) Hoping he is on the mend and surrounded by loved ones #MuhammadAli @MuhammadAli."

In recent years, Ali had largely stayed out of the public spotlight. In his last known appearance, Ali appeared at Parkinson's fundraiser April 9 in Phoenix, according to the Arizona Republic. A photo posted by the newspaper showed Ali wearing dark sunglasses.

He once was known not only for his athletic prowess as a three-time heavyweight champion but also for his social activism.

Ali was born in Louisville, Kentucky, as Cassius Clay in January 1942. He began boxing as an amateur when he was 12 years old and in 1964 became heavyweight champion with a knockout of Sonny Liston. That year he joined the Nation of Islam and changed his name.

Ali's sparkling career was interrupted for 3½ years in the 1960s when he refused induction into the U.S. Army during the Vietnam War and was convicted of draft evasion. The Supreme Court overturned the conviction.

He went on to win the heavyweight title twice more before retiring for good in 1981.