The 74-year-old boxing hero was admitted to a medical centre in Phoenix, Arizona, for respiratory complications earlier this week (beg30May16), and reports suggest he spent his final few hours on life support.
Family spokesman Bob Gunnell confirmed the sad news of Ali's passing in a statement released late on Friday.
"After a 32-year battle with Parkinson's disease, Muhammad Ali has passed away at the age of 74," he wrote. "The Ali family would like to thank everyone for their thoughts, prayers, and support and asks for privacy at this time."
Sports stars, actors and musicians flocked to social media to express their condolences after news of his death broke, with many posting photos paying tribute to one of the greatest sportsmen of all time.
Former boxing champion-turned-actor Mike Tyson shared a snap of the pair on Twitter, and captioned it, "God came for his champion. So long great one. @MuhammadAli #TheGreatest #RIP".
Undefeated fighter Floyd Mayweather, Jr. shared a photo of Ali on Instagram surrounded by money in front of a safe, and called the late sports icon a pioneer: "Today my heart goes out to a pioneer, a true legend, and a hero by all means! Not a day went by entering the gym that I didn't think of you. Your charisma, your charm and above all, your class are all of the elements that will be greatly missed by myself and the world.
"You are someone that inspired me greatly throughout my boxing journey and words cannot express how great you were as a person! Thank you for everything you've done for Black America, in the the world of sports & entertainment and for the legacy you leave behind! My sincerest condolences to the Ali family!"
Mayweather's ring rival Manny Pacquiao also honoured Ali's memory on Twitter, hours after sending the ailing star his well-wishes online. He posted, "We lost a giant today. You will always be GOAT (Greatest Of All Time). Our hearts and prayers go out to the Ali family. May God bless them."
Former boxing champion and Olympic gold-medalist George Foreman also wrote a touching message on Twitter, acknowledging two of his boxing peers who had sadly passed away: "Ali, (Joe) Frazier & Foreman we were 1 guy. A part of me slipped away, 'The greatest piece'."
Foreman fought Ali in the famous Rumble in The Jungle match that took place in Zaire - now the Democratic Republic of the Congo - in the summer of 1974. Ali won the fight, becoming the world heavyweight champion.
Meanwhile, Madonna, who only recently grieved the loss of her close friend Prince, shared a photo of herself with Ali on Instagram, with the caption, "What is going on??!! We are losing all of our National Treasures. Our Pillars of Humanity. He was the Greatest!"
"This is the ALI we should all remember!" boxing fan Justin Bieber insisted beside a snap of Ali joking about in front of the camera. "The fun loving man who was fully himself!! RIP CHAMP!!!"
Rap mogul Sean 'Diddy' Combs, who has a tattoo of Ali's signature on his chest, took to Instagram to remember a meeting with his mentor and friend.
"This pic means so much to me!," he wrote. "The love he showed me he didn't have too (sic). He always was excited to see me. He even came to see me on Broadway in A Raisin In The Sun. He sat thru (sic) a 3 hr (sic) play to see me. It bugged me out. It humbled me. Thank you for the love. I MISS YOU. #RIPALI".
The rapper-turned-actor later shared a photo of his son Justin being hugged and kissed by Ali, writing, "Thank you for always showing me and my family love. He was so kind to everyone he met! I want my son @princejdc to always cherish this moment."
Star Wars actor John Boyega also marked Ali's loss on Twitter, commenting, "May our lives be an inspiration to the people around us as much as our words. The greatest. Rest easy champ", while Frozen star Josh Gad tweeted, "In a year where many greats have passed, the greatest of all has just left us. Goodbye champ. Fly like a butterfly."
Hip-hop veteran Chuck D, who narrated the Ali Rap documentary in 2006, was onstage at the Hollywood Palladium with his new rap/rock supergroup Prophets of Rage on Friday when the news of Ali's death hit headlines, and he called on fans to salute Ali by holding up their fists.
"We raise a fist and pray for Muhammad Ali," he told the crowd, who willing obliged.
Online tributes have also poured in from the likes of Jennifer Hudson, Russell Simmons, Kelly Rowland, Ciara, Lenny Kravitz, and Kate Hudson, among many others.
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