He was that rarest of figures, a sportsman that transcended his sport. Muhammad Ali's enormous impact on global society is evident in the flow of tributes from all corners of the globe and from all walks of life.
Describing Ali's last moments, his daughter Hana said that his heart "wouldn't stop beating" for 30 minutes after his organs failed.
Hundreds of athletes, activists and officials offered condolences and remembrances after Ali's passing.
From the world of boxing:
Bob Arum, who promoted 26 of Ali's fights: "He's the most transforming figure of my time, certainly. He did more to change race relations and the views of people than even Martin Luther King. It was a privilege and an honor for me to know him and associate with him."
Don King, promoter of "Rumble in the Jungle" and "Thrilla in Manila": "Without question his legacy is one that he defied the odds because he stood up for what he believed in and when he was put to the test he took personal harm rather than go against his beliefs and what he stood for."
Filipino boxing champion Manny Pacquiao: "We lost a giant today. Boxing benefited from Muhammad Ali's talents but not nearly as much as mankind benefited from his humanity. Our hearts and prayers go out to the Ali family. May God bless them."
Across the sporting world:
Olympic swimmer Janet Evans: "Passing the Olympic torch to Muhammad to light the cauldron at the Atlanta Games in 1996 was the defining moment of my career, and a memory I will treasure forever, as much as any of the medals I won. As Olympians, our role is to inspire others to achieve their dreams, and no person has ever lived that role more than Muhammad Ali."
Tennis star Serena Williams lamented the lost of Ali as "someone who really stood up for what he believed in."
"He was my hero. He always will be," she wrote.
Politics and activism:
US President Barack Obama: "His fight outside the ring would cost him his title and his public standing. It would earn him enemies on the left and the right, make him reviled, and nearly send him to jail. But Ali stood his ground. And his victory helped us get used to the America we recognize today."
Spokesman for UN Secretary-General Ban Ki-moon:
"Mr. Ali was far more than a legendary boxer; he was a world champion for equality and peace. With an incomparable combination of principle, charm, wit and grace, he fought for a better world and used his platform to help lift up humanity."
Hollywood actor Michael J. Fox, a fellow sufferer of Parkinson's disease, also paid tribute to Ali on Twitter.
Civil rights campaigner Rev. Jesse Jackson: "He was a champion in the ring, but, more than that, a hero beyond the ring. When champions win, people carry them off the field on their shoulders. When heroes win, people ride on their shoulders. We rode on Muhammad Ali's shoulders."
Former US President Bill Clinton: 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."
President Clinton is set to speak at Ali's funeral in the boxer's hometown of Louisville, Kentucky, on Friday.