The three-time former world champion, widely considered among the greatest heavyweights in the history of the sport, has passed away after being taken to an Arizona hospital with respiratory issues.
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."
He later told the BBC: "We were like one guy - part of me is gone."Until Ali no one said "I'm beautiful" he was royalty, yet common man was his pal. That is beauty. Greatest kind pic.twitter.com/uX7htKHrGc— George Foreman (@GeorgeForeman) June 4, 2016
"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.
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."
And David Beckham shared a photo online, writing: "Impossible is just a big word thrown around by small men who find it easier to live in the world they’ve been given than to explore the power they have to change it. Impossible is not a fact. It’s an opinion. Impossible is not a declaration. It’s a dare. Impossible is potential. Impossible is temporary. Impossible is nothing.”
Former England footballer Gary Lineker tweeted: "The greatest has fought his final round. Muhammad Ali was the most magnificent athlete who transcended his sport."
Argentinian boxer Marcos Maidana, a two-weight former world champion, tweeted: "Muhammad Ali the all-time greatest has left us. Thanks for everything. Go with God."
Shortly after his death, Mike Tyson tweeted: "God came for his champion. So long great one. @MuhammadAli #TheGreatest #RIP "
Former boxing champion Oscar De La Hoya tweeted: "RIP muhammadali, a legend who transcended sport and was a true champion for all. #thegreatest #MuhammadAli."
And supermodel Naomi Campbell shared: "not the #greatest please no #may #muhammadali #rip #this such a sad day #hero to all of us".RIP @muhammadali, a legend who transcended sport and was a true champion for all. #thegreatest #MuhammadAli pic.twitter.com/vjmXlXaHip— Oscar De La Hoya (@OscarDeLaHoya) June 4, 2016
Meanwhile, reality stars, musicians and actors have all added their voices to the stream of tributes.😪😪😪 not the #greatest please no #may #muhammadali #rip #this such a sad day #hero to all of us ❤️❤️❤️🙌🏾🙏🏾🌹🌹🌹 pic.twitter.com/GPg5rLiFbM— Naomi Campbell (@NaomiCampbell) June 4, 2016
Kendall Jenner shared a photo of him in the ring, and wrote: "What a legend. RIP."
And her father Caitlyn Jenner added: "Ali, you were generous every time I met you. Thank you for all you did for so many. Your memory will live on forever. #Olympicgold #GOAT."what a legend. RIP pic.twitter.com/JwcwTf3uzO— Kendall (@KendallJenner) June 4, 2016
While Kris Jenner wrote: "Rest in peace, champ. #thegreatest."
Ali, you were generous every time I met you. Thank you for all you did for so many. Your memory will live on forever #Olympicgold #GOAT— Caitlyn Jenner (@Caitlyn_Jenner) June 4, 2016
And 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.Rest in peace, champ. #thegreatest pic.twitter.com/PAABp2QgwR— Kris Jenner (@KrisJenner) June 4, 2016
"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."
Ali, who was diagnosed with Parkinson's Disease 32 years ago, is widely considered to be one of the greatest heavyweights in boxing history.
A statement read: "The Ali family would like to thank everyone for their thoughts, prayers, and support and asks for privacy at this time."God came for his champion. So long great one. @MuhammadAli #TheGreatest #RIP pic.twitter.com/jhXyqOuabi— Mike Tyson (@MikeTyson) June 4, 2016
His biographer Thomas Hauser told the BBC that Ali was "beautiful inside and beautiful outside". He tweeted that Ali's death was "the passing of a legend".
Retired Irish boxer Barry McGuigan spoke about Ali's actions inside and outside of the ring, including his role in the civil rights movement and his refusal to fight in the Vietnam war.
He said he was "a remarkable human being".
"He was hugely inspirational for me and many fighters all over the world," he said.
"Everybody wanted to box because of him."
He said: "He was just so amazing in every way. More than anything else it was how humble and how brilliantly charismatic he was.
"He was a beautiful looking man, a beautiful looking individual and he had so much compassion."
He added: "He was the greatest sportsman there has ever been and we were very lucky that he chose boxing."
Ali's funeral will take place in his hometown of Louisville, Kentucky.
Earlier his daughter, Laila Ali, shared a touching photo on social media and thanked fans for their support.
The 38-year-old posted on her official Facebook page: "I love this photo of my father and my daughter Sydney when she was a baby!
"Thank for all the love and well wishes. I feel your love and appreciate it!!"