The world is invited to the funeral of Muhammad Ali in his hometown on Friday, where the boxing legend's life will be celebrated with a public funeral procession and memorial service, a family spokesman said.

Ali, a three-time world heavyweight champion and civil rights activist who was an iconic figure of the 20th century, died Friday aged 74 after health problems complicated by a long battle with Parkinson's disease.

The official cause of death was septic shock due to unspecified natural causes.

The dazzling fighter - whose words, often delivered in catchy rhymes, were as devastating as his punches - had been admitted to an Arizona hospital earlier in the week.

Political leaders, sports figures, celebrities and fans around the world paused to remember "The Greatest," whose career spanned three decades.

On Sunday, Ali's relatives will accompany his body from Scottsdale, Arizona to Louisville, his hometown in the southern state of Kentucky.

After a private family funeral on Thursday, Ali's coffin will be transported through the streets of Louisville on Friday, before a public memorial service at an arena, with former president Bill Clinton among celebrities expected to offer eulogies.

The procession has been organized to "allow anyone that is there from the world to say goodbye," family spokesman Bob Gunnell told reporters.

Louisville lowered flags to half-staff in his honor, as fans flocked to the boxer's modest childhood home, now a museum, to pay their respects.

President Barack Obama led tributes for Ali, issuing an unusually personal statement in which he said he keeps a pair of Ali's boxing gloves and a photo in his private study.

"Muhammad Ali was The Greatest. Period," Obama said.

"His fight outside the ring would cost him his title and his public standing. It would earn him enemies on the left and the right, make him reviled and nearly send him to jail. But Ali stood his ground," Obama said.

In a possible preview of Bill Clinton's eulogy, he and his wife Hillary, who is seeking the Democratic presidential nomination, said Ali was "a blend of beauty and grace, speed and strength that may never be matched again."

Ali will be buried at Cave Hill Cemetery in Louisville, where he was born in 1942.