Tributes have flooded in from the sports world and beyond for Muhammad Ali.
US President Barack Obama and his wife Michelle issued a statement paying their respects to Ali.
"Muhammad Ali was The Greatest. Period. If you just asked him, he'd tell you. He'd tell you he was the double greatest; that he'd 'handcuffed lightning, thrown thunder into jail'," the statement said.
"But what made The Champ the greatest - what truly separated him from everyone else - is that everyone else would tell you pretty much the same thing.
"Like everyone else on the planet, Michelle and I mourn his passing. But we're also grateful to God for how fortunate we are to have known him, if just for a while; for how fortunate we all are that The Greatest chose to grace our time," added the statement.
David Beckham posted a photograph of himself meeting Ali and called him: "the greatest there will ever be...the biggest and the best."
Former heavyweight boxing champion Evander Holyfield told MSNBC: "I'm glad to have known Ali because when I was a kid, at eight years old, I was told I would be like Ali.
"To take it upon yourself and say; 'I'm the greatest', you put yourself in a position for people to take pot shots at you. This is what Ali did. It's amazing him becoming three-time heavyweight champion of the world. At that time people thought, 'Who could beat three?
"You have to be stronger to get up from a loss to go on and that's what Ali proved to be."
Muhammad Ali - An Obituary
Muhammad Ali - In his own words
Former heavyweight champion Mike Tyson tweeted: "God came for his champion. So long great one. @MuhammadAli #TheGreatest #RIP."
Ali's former promoter Don King told CNN: "He's always been right there, Johnny on the spot, anything he could do for the benefit of mankind.
"Let us celebrate his life. This is not a time to mourn. This is a time to try to emanate the job he was doing and the burden he leaves behind for us to carry on, to remember that the people are the most important."
Boxing champion Floyd Mayweather Jr told Fox News: "There will never be another Muhammad Ali. The black community all around the world, black people all around the world, needed him. He was the voice for us. He's the voice for me to be where I'm at today.
"I just want to thank Muhammad Ali and his family for being such strong people. You will always be missed. My prayers go out to Ali and his family."
George Foreman, Ali's friend and rival from the famous "Rumble in the Jungle" fight, told the BBC: "We were like one guy - part of me is gone."
"Muhammad Ali was one of the greatest human beings I have ever met. No doubt he was one of the best people to have lived in this day and age. To put him as a boxer is an injustice."
He also spoke of Ali's love for the UK and the way he was treated in the country.
"He loved London. If he had been born and raised in London he never would have changed his name," he said.
President Michael D Higgins said: "The passing of Muhammad Ali will have been heard with the greatest sadness by Irish people of all generations.
"Many will remember the wit, grace and beauty he brought to boxing and some will recall his visits to Ireland.
"All over the world people also flocked to hear him offer his view on the achievement of democracy and particularly equal rights when they were so strikingly missing in some of the richest countries of the world.
"He brought his message of freedom and respect for people of all races to all the continents of the world.
"As a sportsman and humanitarian, and as someone who struggled for a very long time with one of the most debilitating illnesses, he offered courage in the face of great difficulties.
"He was intent on going on communicating right to the very end.
"He will be missed not only by his close friends and family but by those who heard him and were influenced by him, all over the world."
A Book of Condolences is to be opened by Clare Co council in Ennis on Tuesday.
In 2009 Ali visited the town along with his daughter for the unveiling of a statue marking his ancestral link with the area.
His great grandfather Abe Grady was from the Ennis area. Ali still has distant relatives in Co Clare.
Minister for Sport Shane Ross said: "I want to add my sincere condolences to the Ali family and the entire boxing community who have today lost 'The Greatest'. He made the entire world listen, his charm and charisma inspired people around the world, and today the world mourns.
"He is arguably the first superstar sportsman, and will always be people's number one. Some were lucky enough to watch him fight in Croke Park and he [lit] up the country when he came and visited, as he did wherever he went."
British Prime Minister David Cameron tweeted: "Muhammad Ali was not just a champion in the ring - he was a champion of civil rights, and a role model for so many people."
Former US president Bill Clinton said: "Hillary and I are saddened by the passing of Muhammad Ali. From the day he claimed the Olympic gold medal in 1960, boxing fans across the world knew they were seeing a blend of beauty and grace, speed and strength that may never be matched again."
Rev Jesse Jackson told BBC Radio 4's Today programme: "He became a champion inside the ring and a hero outside. Champion because he won the boxing matches, hero because he stood up against the war in Vietnam."
Michael Parkinson, who had a series of famously explosive interviews with the boxer, told Today: "It's one thing to be abused by a film star but to be abused by a man weighing 17 stone who is the heavyweight champion of the world, who would kill you with one blow, your options are a bit limited."
Ringo Starr, who met the boxer along with the other members of The Beatles in 1964, tweeted: "God bless Muhammad Ali peace and love to all his family."
