Thousands of people gathered to celebrate Muhammad Ali's life at a traditional Islamic prayer service in the boxer's home town of Louisville, Kentucky.
Members of Ali's family joined former world heavyweight boxing champion Lennox Lewis, Ali's former manager Don King and Reverend Jesse Jackson among the mourners at Freedom Hall, where Ali made his professional debut with victory over Tunney Hunsaker in 1960.
Up to 14,000 tickets were released for the event, with Ali fans travelling from around the world to honour the three-time world heavyweight boxing champion.
Briton Abi Ajram, 48, said he had made the 4,000-mile trip from London to pay his respects to "a great man", who died last Friday aged 74.
He told the Press Association: "I feel Muhammad Ali deserved the world turning up for him. I thought it's the least I can do and I wanted to do it the moment I heard he had passed away.
"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.
"I met him at the Royal Albert Hall in 2000. I shook his hand. I didn't know what to say."
Ali's coffin was wheeled into the building for the service, known as a Jenazah, which was attended by Turkish president Recep Tayyip Erdogan, former boxing champion Sugar Ray Leonard and Nation of Islam leader Louis Farrakhan.
Ali - who battled Parkinson's disease for 32 years - joined the Nation of Islam, the black separatist religious movement, in the 1960s, but left after a decade to embrace mainstream Islam.
Addressing the crowd, Muslim scholar Sherman Jackson said: "For millions of people around the world of every race, religion and political persuasion, the passing of Muhammad Ali has made us feel a little more alone in the world.
"I can hardly begin to imagine the depths of his family's sense of loss.
"Ali was an unapologetic fighter in the cause for black people in America."
Speaking outside Freedom Hall, King told reporters: "Four years in exile at the height of his career, nobody can really say how great Muhammad Ali would have been."
About 15,000 people are expected to attend the huge public memorial at the KFC Yum! Centre after tickets quickly sold out on Wednesday.
But some ticket-holders have been criticised for trying to profit from the free event by selling tickets online.
Ali family spokesman Bob Gunnell said: "I'm personally disgusted and amazed that someone would try to profit off of Muhammad Ali's memorial service. I hope that those buying tickets or trying to buy tickets would stop those efforts by not purchasing.
"Muhammad Ali wanted this to be a free event, an event that was open to all."
Lewis, who will be one of the pallbeares at the funeral, said: "It's such a privilege and such an honour to be able to send the greatest and say farewell to him. He's in a better place. He will never be forgotten.
"He was absolutely the greatest. He was the only one that floated like a butterfly and stung like a bee."
Lewis will be a pallbearer alongside Hollywood actor Will Smith, who portrayed Ali in the 2001 film about the boxer's life.
The others will be 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.
Two of Ali's daughters, Rasheda and Maryum, will speak at the funeral, along with Ali's widow Lonnie, former US president Bill Clinton, American actor Billy Crystal and Malcolm X's daughter Attalah Shabazz.
The service will be live-streamed on the internet and can also be seen at a screening at London's 02 Arena.