(CNN) Muhammad Ali, the legendary boxer who proclaimed himself "The Greatest" and was among the most famous and beloved athletes on the planet, died Friday in Arizona, a family spokesman said.

Ali had been at HonorHealth Scottsdale Osborn Medical Center in Scottsdale since Thursday with what spokesman Bob Gunnell had described as a respiratory issue.

"After a 32-year battle with Parkinson's disease , Muhammad Ali has passed away at the age of 74. The three-time World Heavyweight Champion boxer died this evening," Gunnell said in a statement. "The Ali family would like to thank everyone for their thoughts, prayers, and support and asks for privacy at this time."

Ali's daughter Hana Ali said her father was a "humble mountain."

"And now he has gone home to God. God bless you daddy. YOU ARE THE LOVE OF MY LIFE!" she tweeted.

Our father was a "Humble Mountain!" And now he has gone home to God. God bless you daddy. YOU ARE THE LOVE OF MY LIFE! — Hana Ali (@Hanayali) June 4, 2016

In a separate tweet, Hana Ali said his children surrounded Ali in his final moments, holding his hands, hugging and kissing him and chanting Islamic prayer. She wrote that some whispered in his ear, "You can go now. We will be okay. We love you. Thank you. You can go back to God now."

After Ali's organs failed, his daughter wrote in the tweet, his heart continued to beat for another 30 minutes: "A true testament to the strength of his Spirit and Will!"

Longtime Ali friend John Ramsey, who works for WAVE in Louisville, Kentucky -- Ali's hometown -- and who has a radio show on ESPN, was at the hospital with Ali's family when the boxing legend died.

"When he came into the hospital, we thought, 'OK, it will be a brief stay'... I think it took a turn for the worse," he told CNN's Dan Simon. "But it was unexpected."

Ramsey said he didn't actually see Ali in the hospital. The people who were with Ali when he died were "the kids, family only, and I respected that... it's a very private matter."

Ramsey said Ali's wife, Lonnie, called him Friday morning, saying Ali's health was deteriorating.

"She said 'You might want to come out,' which I decided to do," he said.

Tributes from around the world

While touching tributes to Ali were pouring in from world leaders, fellow athletes and just regular folk, the boxing great had already addressed how he wanted the world to think about him after his death.

In his book "The Soul of a Butterfly: Reflections on Life's Journey," Ali said he wanted to be remembered as "a man who won the heavyweight title three times, who was humorous, and who treated everyone right. As a man who never looked down on those who looked up to him, and who helped as many people as he could. As a man who stood up for his beliefs no matter what. As a man who tried to unite all humankind through faith and love."

He added, " And if all that's too much, then I guess I'd settle for being remembered only as a great boxer who became a leader and a champion of his people, And I wouldn't even mind if folks forgot how pretty I was."

President Barack Obama said he and first lady Michelle Obama mourn Ali's 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," the Obamas said in a statement.

The President said he has a pair of Ali's gloves on display in his private study off of the Oval Office, beneath a photograph of the legend.

Obama said Ali "fought for us."

"He stood with King and Mandela; stood up when it was hard; spoke out when others wouldn't. 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," the President said in a statement issued by the White House.

His hometown pauses to reflect

Louisville Mayor Greg Fischer, at a ceremony honoring Ali on Saturday, said the boxer "lived a life so big and bold, it's hard to believe that any one man could do everything he did, could be all the things that he became in the course of just one lifetime."

Fischer added, "Muhammad Ali belongs to the world, but he only has one hometown. The 'Louisville Lip' spoke to everyone, but we heard him in a way no one else could -- as our brother, our uncle, and our inspiration."

Don King, the boxing promoter who was every bit as brash as Ali, told CNN that in his mind Ali will never die.

"His spirit will go on forever," he said. "He's just a great human being, a champion of the people, the greatest of all time."

Even as the former champ battled Parkinson's disease, he had the same love for life and people, King said. Parkinson's, which primarily affects a patient's movement, is a "progressive disorder of the nervous system," according to the Mayo Clinic.

The Ali sense of humor was displayed in an old photo tweeted by Oscar De La Hoya, who won titles in six weight classes.

"RIP @muhammadali, a legend who transcended sport and was a true champion for all. #thegreatest #MuhammadAli," De La Hoya wrote above an image of Ali whose mischievous eyes proudly watched as his right hand made rabbit ears behind the "Golden Boy's" head.

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

Hours before her famed father passed away, Laila Ali posted a throwback photo of him with her daughter, Sydney, who was born in 2011.

"I love this photo of my father and my daughter Sydney when she was a baby! Thanks for all the love and well wishes. I feel your love and appreciate it!!" Laila Ali, herself a former world champion boxer, wrote.

George Foreman, who Ali defeated in 1974 for the world heavyweight title, wrote on Twitter, "It's been said it was rope a dope Ali beat me with. (N)o (it was) his beauty that beat me. Most beauty I've know(n). loved him."

It's been said it was Rope a dope, Ali beat me with no his beauty that beat me. Most beauty I've know loved him pic.twitter.com/G64WX3eyZC — George Foreman (@GeorgeForeman) June 4, 2016

Famed promoter Bob Arum wrote on Twitter: "A true great has left us. @MuhammadAli transformed this country and impacted the world with his spirit."

A true great has left us. @MuhammadAli transformed this country and impacted the world with his spirit. pic.twitter.com/ByNZYhM2ro — Bob Arum (@BobArum) June 4, 2016

Mike Tyson, the youngest heavyweight champion in history, said, "God came for his champion. So long great one. @MuhammadAli #TheGreatest #RIP."

Another hero gone. Rest In Power Muhummad Ali. The Greatest. pic.twitter.com/JO8EoaERD7 — Ice Cube (@icecube) June 4, 2016

Champion for Parkinson's families

In recent years, Ali had largely stayed out of the public spotlight. In his last known appearance, he appeared at a Parkinson's fundraiser April 9 in Phoenix, according to the Arizona Republic. A photo posted by the newspape r showed Ali wearing dark sunglasses.

On the website of the Michael J. Fox Foundation, the actor -- who also has Parkinson's -- said: "Muhammad was a true legend -- a champion in the boxing ring, and a champion for millions of Parkinson's families. We looked up to him as an example of grace and courage in the face of great challenges."

Ali was known not only for his athletic prowess but also for his social activism.

He was born in Louisville, Kentucky, as Cassius Clay in January 1942. He began boxing as an amateur when he was 12 years old and in 1964 became heavyweight champion with a knockout of Sonny Liston. That year he joined the Nation of Islam and changed his name.

Ali's sparkling career was interrupted for 3½ years in the 1960s when he refused induction into the U.S. Army during the Vietnam War and was convicted of draft evasion. The Supreme Court overturned the conviction.

Ali was prepared to go to prison, King said.

Spent so much of my life preoccupied with Muhammad Ali; watching, reading, thinking and talking about him. I was one of millions. — Max Kellerman (@Max_Kellerman) June 4, 2016

"He stood his ground on who he was," King said. "He'd rather go to jail than break what he believed in."

During his boxing hiatus, Ali spoke out about racism in America.

NBA legend Kareem Abdul-Jabbar called Ali a friend and a mentor.

"At a time when blacks who spoke up about injustice were labeled uppity and often arrested under one pretext or another, Muhammad willingly sacrificed the best years of his career to stand tall and fight for what he believed was right," Abdul-Jabbar wrote on Facebook.

Ali went on to win the heavyweight title twice more before retiring for good in 1981.

Ali also was hospitalized in January 2015 with a urinary tract infection. He was hospitalized in December 2014 with pneumonia.

Three off-duty Louisville police officers were to escort Ali's body from Arizona to Kentucky, where funeral services will be held, according to police spokeswoman Alicia Smiley.