World prepares to say goodbye to legend Muhammad Ali
World leaders and celebrities will be among 18,000 people expected to attend the service in Ali's home city of Louisville, Kentucky, with the ceremony broadcast to millions of viewers around the world.
Former US president Bill Clinton will a deliver a eulogy, while Ali's widow Lonnie and two of his nine children, daughters Rasheda and Maryum, will also address the congregation.
Lennox Lewis, the former world heavyweight boxing champion, and Hollywood actor Will Smith, who portrayed Ali in the 2001 film about the boxer's life, will be pallbearers.
Speaking ahead of the funeral, Lewis said it was a "privilege" and an "honour" to be involved in the ceremony.
He told the Press Association: "We're laying to rest another king - the greatest.
"He was the only one that floated like a butterfly and stung like a bee. I think he's in a better place right now."
Fans from around the world have flocked to Louisville for the funeral, which is being held at the KFC Yum! Centre at 2pm local time (7pm BST).
Briton Abi Ajram, 48, said he had made the 4,000-mile trip from London to pay his respects to "a great man".
He told the Press Association: "I feel Muhammad Ali deserved the world turning up for him.
"He was the number one. I wanted to show my respect to a great man and an even greater humanitarian. He meant a lot to me personally."
A procession will take place through the streets of Louisville in the morning before a private burial at Cave Hill Cemetery.
US president Barack Obama, who will miss the funeral to attend his daughter Malia's high school graduation ceremony, described Ali as "an icon" and a "personal hero" who transformed "not just the world of sport, but the world as a whole".
"He was one of a kind. In my book, he'll always be the greatest."
Mr Obama and his wife Michelle are sending a letter to be read at the service, where White House senior adviser Valerie Jarrett - who knew Ali personally - will represent the president.
American actor Billy Crystal and Malcolm X's daughter Attallah Shabazz will speak at the funeral, while Turkish president Recep Tayyip Erdogan and King Abdullah II of Jordan are expected to be among the congregation .
The pallbearers include Jerry Ellis - the brother of Jimmy Ellis, Ali's former sparring partner and fellow world heavyweight champion - and Ali's cousins John Grady and Jan Wadell, nephew Ibn Ali, former brother-in-law Komawi Ali and family friend John Ramsey.
The funeral will be live-streamed on the internet and is also being shown at London's 02 Arena, the site of his I Am The Greatest Exhibition.
Some ticket-holders for the funeral have been criticised for trying to profit from the free event by selling tickets online.
Ali family spokesman Bob Gunnell said he was "personally disgusted" at attempts to profit from Ali's funeral and urged people not to buy the tickets.
A traditional Islamic prayer service was held for Ali on Thursday at Freedom Hall, where Ali made his professional debut with victory over Tunney Hunsaker in 1960.