Former heavyweight boxing champion Muhammad Ali has died, according to a family spokesperson cited by ABC and NBC News.

Ali had been suffering from respiratory issues this week, apparently related to his Parkinson's disease, and was hospitalized in Arizona, according to sources cited by Reuters.

"It's extraordinarily grave. It's a matter of hours," the source told the newswire service earlier Friday.

Ali, 74, was diagnosed with Parkinson's in the 1980s, but remained active for much of the past 30 years, despite challenges caused by the neurological disease.

News of Ali's rapidly deteriorating health Friday marked a significant downturn for the former boxer who's condition was described by doctors as "fair" the day before.

Ali has been hospitalized several times over the years, most recently in early 2015. The Hollywood Reporter noted that he has looked "increasingly frail" in public appearances, "including April 9 when he wore sunglasses and was hunched over" during an event in Phoenix.

Ali's daughter, Laila, who was an undefeated boxer herself, was reportedly among Ali's nine children with him at the hospital in Arizona. She thanked fans for their support and posted a baby picture of her daughter, Sydney, who was born in 2011, being held by the boxing legend.

Despite the debilitating effects Parkinson's disease had on Ali's body, it never seemed to daunt the boxing legend's spirit. Ali remained a charismatic presence in public.

A towering figure in his prime, he still traveled and made appearances in his later years despite being muted by the thousands of punches he took during his remarkable career.

He beat the invincible Sonny Liston, fought a string of thrilling fights with Joe Frazier and stopped George Foreman in the "Rumble in the Jungle" in Zaire. But he paid a terrible price for the estimated 29,000 punches he took to his head during a career that made him perhaps the most recognized person in the world.

The Associated Press contributed to this report