Los Angeles, (AFP/PTI) Heavyweight boxing legend Muhammad Ali, an icon of the 20th Century whose fame transcended the sport during a remarkable career that spanned three decades, has died, his family said.

The beloved 74-year-old sports hero, who had been battling Parkinson's disease for decades, passed away yesterday in a hospital in Phoenix, Arizona, where he had been admitted earlier this week suffering from respiratory problems.

"After a 32-year battle with Parkinson's disease, Muhammad Ali has passed away at the age of 74," spokesman Bob Gunnell said.

"The three-time World Heavyweight Champion boxer died this evening."

Funeral arrangements for Ali would be announced today, he added.

Concern for Ali had grown throughout Friday, and tributes immediately poured in for "The Greatest," who was known globally not only for his storied ring career but also for his humanitarian activism.

Ali had been hospitalized multiple times in recent years.

He spent time in hospital in 2014 after suffering a mild case of pneumonia and again in 2015 for a urinary tract infection.

His Parkinson's, thought to be linked to the thousands of punches he took during a brutal career studded by bruising battles inside the ropes, had limited his public speaking.

But he continued to make appearances and offer opinions through his family members and spokespeople.

In April, he attended a Celebrity Fight Night Dinner in Phoenix that raised funds for treatment of Parkinson's.

In December, he issued a statement rebuking US presidential hopeful Donald Trump's call for a ban on Muslims entering the United States.

"Muhammad Ali transformed this country and impacted the world with his spirit," said longtime boxing promoter Bob Arum. "His legacy will be part of our history for all time.