Boxing legend and US icon Muhammad Ali has been farewelled in an emotion-charged memorial service.

It’s estimated 100,000 people lined the streets of his hometown of Louisville, Kentucky on a hot and sunny Friday (local time) to honour Ali, with fans hurling flowers and chanting “Ali, Ali”, along the 37km procession route.

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The huge gathering of fans was joined by a cross-section of sports and movie stars, religious and political figures who paid their respects to arguably the greatest sportsman of all time.

Ali died a week ago aged 74, and his passing has underlined his stature as one of the most respected men in the US – a civil rights campaigner who converted to Islam and lost three years in the ring after refusing US military service during the Vietnam War.

Fans gather at Cave Hill graveyard in Louisville. Photo: Getty

As the funeral motorcade wound past his boyhood home, the city’s Muhammad Ali Centre and other important sites in his life, residents held up photos of “The Greatest”, as Ali dubbed himself, while others tried to run or bicycle alongside, as if unwilling to let him go.

After a private burial ceremony at Louisville’s Cave Hill Cemetery, a further 22,000 people packed a public, star-studded memorial service later on Friday.

Hollywood actor Will Smith and former heavyweight boxers Lennox Lewis and Mike Tyson were pallbearers at the funeral, before an inter-faith service was held at the KFC Yum Centre to celebrate Ali’s life.

Pallbearers included Mike Tyson (far left) and Will Smith (far right). Photo: Getty

About 15,000 free tickets were snapped up for the event, which was attended by celebrities including David Beckham and Arnold Schwarzenegger, and broadcast to millions of viewers around the world.

Speaking at the service, former US president Bill Clinton described Ali as a “universal soldier of our common humanity” who fought heroically against Parkinson’s disease.

Akera Christ-King, nine, waits outside Ali’s childhood home for the funeral procession. Photo: AAP

“I will always think of Muhammad as a truly free man of faith,” Mr Clinton told the crowd.

“He refused to be imprisoned by a disease that kept him hamstrung longer than Nelson Mandela was kept in prison in South Africa.”

Will Smith greets fans. Photo: Getty

Sportscaster Bryant Gumbel said: “What does it say of a man, any man, that he can go from being viewed as one of his country’s most polarising figures to arguably its most beloved?”

Ali’s widow Lonnie told the service her family were “humbled” by the “heartfelt expressions of love” from around the world since her husband’s death.

“Muhammad fell in love with the masses and they fell in love with him,” she said.

Wife Lonnie Ali: “If Muhammad did not like the rules, he rewrote them. His religion, his name, his beliefs, were his to fashion, no matter what the cost.” Photo: AAP

“He did not fear death. He saw the good soul in everyone.”

US President Barack Obama sent a letter to be read at the service as he was attending his daughter Malia’s high school graduation ceremony.

Soccer player David Beckham, left, arrives for Ali’s memorial service. Photo: AAP

In the letter, read by White House senior adviser Valerie Jarrett, the President described Ali as “more original and influential than just about anyone of his era”.

“His jab knocked some sense into us and pushed us to expand our imagination,” President Obama wrote.

Comedian Billy Crystal drew laughter from the crowd when he impersonated Ali on stage in a moving tribute.

Arnold Schwarzenegger was among the mourners. Photo: AAP

“Thirty-five years after he stopped fighting he is still the champion of the world,” Crystal said.

“He was the most perfect athlete you ever saw … and those were his own words.

“He taught us life is best when we build bridges between us, not walls.”

Crystal also imitated sportscaster Howard Cosell, who backed Ali in the fighter’s early years.

“Only once in a thousand years or so do we get to hear a Mozart, or see a Picasso, read a Shakespeare. Ali was one of them,” Crystal said. “And yet, at his heart, he was still a kid from Louisville who ran with the gods and walked with the crippled and smiled at the foolishness of it all. He is gone, but he will never die.”

Two of Ali’s nine children, daughters Maryum and Rasheda, also spoke at the service.

Former President Bill Clinton: “We should honour him by letting our gifts go among the world as he did.” Photo: AAP

In a direct message to her father, Rasheda said: “You were the greatest father to us. It was God’s will to take you home.

“You shook up the world in life. Now you’re shaking up the world in death. Until we meet again, fly butterfly, fly.”

Dr Kevin Cosby, a Protestant minister in Louisville, delivered a rousing speech as he told the congregation: “Before James Brown said, ‘I’m black and I’m proud’, Muhammad Ali said ‘I’m black and I’m pretty’. Black and pretty was an oxymoron.

“He dared to love black people at a time when black people had problems loving themselves.”

Cosby likened Ali to other ground-breaking black athletes who advanced civil rights, such as baseball player Jackie Robinson, boxer Joe Louis and track star Jessie Owens.

Attallah Shabazz, daughter of slain Nation of Islam leader Malcolm X, added: “Having Muhammad Ali in my life somehow sustained my dad’s breath for me just a little while longer — 51 years longer until now.”

The funeral procession and memorial marked the second day of services honouring Ali.

A traditional Islamic prayer ceremony – known as a jenazah – was held on Thursday at Freedom Hall, where Ali made his professional debut with victory over Tunney Hunsaker in 1960.

– with AAP