Muhammad Ali made his final journey through his home town in a funeral procession as thousands of mourners lined the streets where the future heavyweight champion of the world once chased school buses in hiking boots to train for his fights.

His cherry-red coffin, draped in an Islamic shroud, was loaded into a hearse as a group of pallbearers including former boxers Mike Tyson and Lennox Lewis and actor Will Smith left the funeral home in a double file.

Ali's nine children, wife, two ex-wives and other family members joined the motorcade.

The 17-car procession set out for a Louisville cemetery on a 19-mile route expected to take Ali's body past his boyhood home, the gym where he learned to box and the museum that bears his name, by way of Muhammad Ali Boulevard.

He is to be laid to be rest under a headstone inscribed simply "Ali", in a private graveside ceremony, followed in the afternoon by a memorial service attended by more than 15,000 people, including former president Bill Clinton and comedian Billy Crystal.

Ali, the most magnetic and controversial athlete of the 20th century, died last Friday at 74 after a long battle with Parkinson's disease. A traditional Muslim funeral service was held on Thursday, with about 6,000 admirers from all over the world.

Ali chose the cemetery, which is on the National Register of Historic Places, as his final resting place a decade ago. Its 130,000 graves represent a who's who of Kentucky, including KFC founder Colonel Harland Sanders.

Family spokesman Bob Gunnell said the simple stone in Cave Hill Cemetery will be in keeping with Islamic tradition.

Ali decided years ago that when he died, the funeral would be open to ordinary fans, not just VIPs. As a result, thousands of free tickets were made available and were snatched up within an hour.

Louisville is accustomed to being in the limelight each May during the Kentucky Derby, but the send-off for the three-time heavyweight champion and global advocate for social justice loomed as one of the city's most historic events.

"We've all been dreading the passing of the champ, but at the same time we knew ultimately it would come," mayor Greg Fischer said. "It was selfish for us to think that we could hold on to him forever. Our job now, as a city, is to send him off with the class and dignity and respect that he deserves."

Tyson was added at the last moment to the list of pallbearers.

Rumours that Donald Trump would attend were quashed when the Republican presidential candidate reportedly called Ali's wife Lonnie to inform her he was unable to make it.

President Barack Obama is unable to make the trip because his daughter Malia is graduating from high school.