Tributes have flooded in from the sports world and beyond for Muhammad Ali.
George Foreman, Ali's great friend and rival from the famous "Rumble in the Jungle" fight, said: "The greatest honour, when I look back: everybody says 'I knew Muhammad, and I know this', but those eight rounds we spent together in Africa (were) probably as close as anyone who's alive has been with him, and I'll never forget that.
Speaking to Press Association Sport, Foreman, 67, revealed how much of an influence Ali had on his life.
"It's so many years now since right before the Olympics, and during the Olympics, I did an interview, and I recited poems, and I thought, 'Maybe Muhammad Ali will look at this, maybe he will hear this and see this'," he said.
"Even when I fought in the Olympics (I thought) the same.
"For all those years, since 1967, I've had my eye pointed to where he may be. I woke up this morning and it wasn't there, so there's a great emptiness in my life."
Ali's death leaves Foreman as the sole significant survivor of the golden age of the heavyweights in which Joe Frazier, Jimmy Young, Ron Lyle, Ken Norton and Jimmy Ellis were also prominent.
"Each time one of us leaves, I tell everybody: Muhammad Ali, Joe Frazier, George Foreman, we were really just one guy," Foreman said. "And every time one slips away, you feel like you've lost a piece, and Muhammad Ali was the greatest piece of all.
Sugar Ray Leonard, another fighter but from the post-Ali era, said: "I woke up this morning with a tear coming down my cheek, an ache in my chest along with an appreciation of a man, fighter and friend that I truly admired, idolised and loved in Muhammad Ali.
"My true feelings have not totally surfaced yet because no one beats Muhammad Ali. So to continue his journey I will thank God for bringing this incredible man into my life. RIP Champ."
Actor Will Smith played Ali in an eponymous film in 2001. Posting photos of the pair in the ring together on his Facebook page, Smith said simply: "You shook up the world! My mentor and my friend. You changed my life. Rest in peace."
Rock legend Rod Stewart paid tribute to sporting icon Muhammad Ali as he kicked off his 2016 UK stadium tour in Norwich on Saturday.
Rod sang a stirring rendition of his 1972 hit Angel as giant pictures of boxing great Ali were beamed onto screens behind him at a sell-out Carrow Road.
The Rev Jesse Jackson, a fellow civil rights campaigner, 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 famous interviews with the boxer, told Today: "We mustn't deify him at all from that point of view, he was a man of many flaws, but he was a man of great genius, great charm, great humour and he was, in his quiet moments, fascinating."
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."
Speaking to 5 News, Frank Bruno said it was a combination of Ali's looks, charisma and boxing that made him so good, and that meeting him made you feel "warm and tingly".
He said: "He could fight, he could box, he had a lot of determination and will power and a lot of psychology he used on boxers, but he was a very, very intelligent guy."
Asked whether he was the greatest boxer, Bruno said: "Yes, I think so. He could box, he could fight, he could suss you out. I think definitely, pound for pound, all the way round.
"There are some boxers who have got power, some boxers have got speed, some boxers have got skill. He had everything."
Fellow British boxer Amir Khan said: "I would like to send my deepest condolences and thoughts to his family at this time. No fighter or sportsman will ever reach the level of Muhammad Ali, whose name will continue to echo through the ages. Inspiring, charismatic, a true legend - Ali will never be forgotten.
"Having the chance to meet the great man will be a memory and privilege I will always hold dear to me - an incredible human being, fighter and role model. Thank you Muhammad for inspiring us all."
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."
Former heavyweight champion Mike Tyson tweeted: "God came for his champion. So long great one", while 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.
Former heavyweight world champion Lennox Lewis said Ali's "courage to stand up for his beliefs, his people, and his convictions" separated him from the rest.
He said: " Ali fought battles that I will never know, so those that came after him, would never have to. He paved the way for the success of all of today's sports superstars and instilled a sense of pride and responsibility inside of me, and generations of youth, around the world.
"He was my source of inspiration, and along with my other hero, Nelson Mandela, served as a shining example of grace, dignity, and courage, through a lifetime of service to others."
Former heavyweight champion David Haye, who named his son Cassius after Ali, revealed it was the late Muhammad Ali who inspired the finest victory of his career.
"When I was training to fight Nikolai Valuev, my first heavyweight title fight, I thought to myself, 'How would Muhammad Ali have fought this guy? What would he have done? What would his tactics have been?'," Haye told Press Association Sport.
"And I really sat down and thought about it, watched lots of tapes of Muhammad Ali. And his movement. I really, truly implemented that.
Tributes also came from figures outside the world of boxing.
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."
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."
David Beckham posted a photograph of himself meeting Ali and called him "the greatest there will ever be ... the biggest and the best".
Muhammad Ali met the Beatles in 1964 when the Fab Four flew to Miami to appear on The Ed Sullivan Show.
While Beatlemania was well under way, Ali - then still Cassius Clay - was an underdog ahead of his world title fight against Sonny Liston.
Sir Paul McCartney paid his own tribute to Ali following his death, saying: "Besides being the greatest boxer, he was a beautiful, gentle man with a great sense of humour who would often pull a pack of cards out of his pocket, no matter how posh the occasion, and do a card trick for you.
"The world has lost a truly great man."
Brazilian football great Pele, who is also regarded along with Ali as one of the greatest sports stars of all time, wrote on Instagram: "The sporting universe has just suffered a big loss.
"Muhammad Ali was my friend, my idol, my hero. We spent many moments together and always kept a good connection throughout the years. The sadness is overwhelming. I wish him peace with God. And I send love and strength to his family."