United States president Barack Obama paid tribute to Muhammad Ali as the legendary boxer and social justice pioneer was farewelled at a memorial service in Louisville, Kentucky on Friday.

Esteemed dignitaries, celebrities and athletes flocked to Ali's hometown to pay their respects to the global icon, who died last week aged 74 following a 32-year battle with Parkinson's disease.

Obama was not in attendance due to scheduling but the two-term serving president did prepare a touching statement, reflecting on Ali's contribution to the world both inside and outside the boxing ring.

"He was Muhammad Ali, a whole greater than the sum of its parts. He was bigger, brighter, more original and influential than just about anyone of his era.  You couldn't have made him up. And yes, he was pretty, too," Obama's statement read.

"He had fans in every city and village and ghetto on the planet; he was feted by foreign heads of state; the Beatles' British invasion took a detour to come to him. It seemed sometimes that The Champ was simply too big for America.

"But I actually think the world flocked to him in wonder precisely because, as he once put it, Muhammad Ali was America.  Brash, defiant, pioneering, joyful, never tired, always game to test the odds.  He was our most basic freedoms – religion, speech, spirit.  He embodied our ability to invent ourselves.  His life spoke to our original sin of slavery and discrimination, and the journey he travelled helped to shock our conscience and lead us on a roundabout path toward salvation. And, like America, he was always very much a work in progress.

"We'd do him a disservice to gauze up his story, to sand down his rough edges, to talk only of floating butterflies and stinging bees.  Ali was a radical even in a radical's time; a loud, proud, unabashedly black voice in a Jim Crow world. His jabs knocked some sense into us, pushing us to expand our imaginations and bring others into our understanding.  There were times he swung a bit wildly, wounding the wrong opponent, as he was the first to admit. But through all his triumphs and failures, Ali seemed to achieve the sort of enlightenment, an inner peace, that we're all striving toward.

"In the '60s, when other young men his age were leaving the country to avoid the war or jail, he was asked why he didn.t join them.  He got angry.  He said he'd never leave – his people are here, the millions 'struggling for freedom, and justice, and equality… I can do a lot to help, in jail or not'.

"He'd have everything stripped from him – his titles, his standing, his money, his passion, very nearly his freedom.  But Ali still chose America. I imagine he knew that only here, in this country, could he win it all back. So he chose to help perfect a union where a descendant of slaves can become the king of the world, and in the process, lend some dignity to all of us – maids, porters, students, maybe even an elderly bathroom attendant – and help inspire a young mixed kid with a funny name to have the audacity to believe he could be anything, even President of the United States. "Muhammad Ali was America. He will always be America.  "What a man. What a spirit. What a joyous, mighty champion. God bless The Greatest of All Time. God bless his family. And God bless the nation we love."