The sporting legend died of septic shock at 11.10pm ET on Friday "due to unspecified natural causes", family spokesman Bob Gunnell said during a press conference on Saturday.

Former world heavyweight champion Ali, who was hospitalised this week with a respiratory issue, passed away aged 74 with his family by his side in a Phoenix-area hospital.

Ex-United States President Bill Clinton, actor Billy Crystal and broadcaster Bryant Gumbel will be among dignitaries giving eulogies at the funeral, which will be open to the public.

The event will begin with a procession through the city, passing locations that were "historically important" to Ali.

Ali's daughter, Hana Ali, posted on Instagram a touching account of her father's final moments Friday night, saying that the family surrounded him and chanted Islamic prayers as the end neared.

She said his heart kept beating for 30 minutes after his other organs failed, and that, "no one had ever seen anything like it."

She called it a "true testament to the strength of his spirit and will."

Gunnell said global icon Ali "did not suffer" and described his final moments with family as "solemn" but "beautiful."

"The champ would have been proud of his family," Gunnell said, adding it was "important to the family to have the celebration open to all walks of life" because Ali "was a citizen of the world."