UPDATED 12.54PM: Muhammad Ali is in "grave condition" in a Phoenix-area hospital, as the boxing legend continues to battle a respiratory condition.

A source close to the family has told Reuters that Ali's condition was extraordinarily grave and it's a matter of hours.

The 74-year-old former heavyweight champion's family has gathered by his bedside just a day after he had been hospitalized in what spokesman Bob Gunnell said on Thursday was to be a "brief" stay.

Ali has been battling advanced Parkinson's disease in recent years - an illness that experts say can lead to complications and affect the breathing of patients.

The boxing legend was previously hospitalized for several days in December 2014 and January 2015 after suffering a severe urinary tract infection initially thought to be pneumonia.

Although weakened by illness, Ali has continued to loom large on the national stage.

In recent months, Ali waded into the nation's political fight after presidential candidate Donald Trump suggested banning all Muslims from entering the U.S.

Ali, who famously joined the Nation of Islam in 1964, asked for more understanding.

Doctors treating the former heavyweight champ told the family they feared he was near the end.