Boxing great Muhammad Ali has died aged 74 in a Phoenix hospital following a short battle with a respiratory illness.

 

Ali, regarded as the greatest professional boxer in the history of the sport, was admitted to hospital earlier this week with the illness and died on Saturday (AEST).

 

He had been suffering from Parkinson's disease for more than three decades.

 

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

 

Ali compiled a 56-5 win-loss record as a professional boxer and won a gold medal at the 1960 Olympics in Rome.

 

Known globally not only for his storied ring career but also for his civil rights activism, Ali had been hospitalised 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.

 

Ali's Parkinson's, thought to be linked to the thousands of punches he took during a career that spanned three decades, had limited his public speaking for years.

 

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.

 

Ali, born in Louisville, Kentucky, as Cassius Marcellus Clay Jr, changed his name in 1964 after his conversion to Islam.

 

His legendary boxing career stretched from 1960 to 1981, dazzling fans with slick moves in the ring, and with his wit and engaging persona outside it.

 

His opposition to the Vietnam War saw him banned from the sport for years, but the US Supreme Court overturned his conviction for draft dodging in 1971.

 

Once vilified in some quarters for his conversion to Islam and his outspoken stance on civil rights issues, Ali held firm to earn dozens of tributes, lighting the Olympic torch in 1996 in Atlanta and being named a UN messenger of peace in 1998.

 

He received the highest US civilian honour, the Presidential Medal of Freedom, in 2005.

 

"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."