Leading the tributes was his ex-wife Lisa Marie Presley.
In a statement reported on the BBC website, she said: "I am so very sad and confused with every emotion possible.
"I am heartbroken for his children, who I know were everything to him, and for his family. This is such a massive loss on so many levels, words fail me."
Mark Lester, who is godfather to Jackson's children, described him as "a very sweet, kind and loving man".
The former child star said the pair first met more than 30 years ago.
"The last time we saw him he was in London a few weeks ago," he said. "He was absolutely fine. I can't believe this, it's such a shock."
He told GMTV that Jackson was ready for his planned gigs in London, saying: "If he didn't think he could do it, he wouldn't have done it."
Former child actor Corey Feldman, who was a close friend of Jackson's, wrote on his website: "He was my idol, he was a role model, he was someone to cry to when my childhood was unbearable, he was a brother, he was a dear friend."
He said the pair fell out eight years ago, but he remained close to the Jackson family.
He wrote: "I am trembling and shaking at the moment and it is very hard to type. I am filled with tremendous sadness and remorse.
"All I choose to remember from this point is the good times we shared and what an inspiration he was to me and the rest of the world.
"Nobody will ever be able to do what Michael Jackson has done in this industry, and he was so close to doing it all again."
A host of celebrities from the music and entertainment industries lined up to acknowledge the star's genius.
Madonna said in a statement on the BBC website: "I can't stop crying over the sad news.
"I've always admired Michael Jackson - the world has lost one of its greats but his music will live on forever.
"My heart goes out to his three children and other members of his family. God bless."
Friend and psychic Uri Geller said from his home in Sonning, Berkshire: "I'm shocked and devastated. I am hoping this is a dream I will wake up from, but it is not. Michael is dead.
"For him not to be around, that he's gone, is just surreal. It cannot sink into my psyche. He was a genius."
He said he feared it may have been the "stress" of Jackson's planned comeback tour that killed him.
Music producer Quincy Jones told MSNBC: "I am absolutely devastated at this tragic and unexpected news."
He said Jackson "had it all - talent, grace, professionalism and dedication", adding: "I've lost my little brother today, and part of my soul has gone with him."
Film directors Martin Scorsese and Steven Spielberg also paid tribute.
Scorsese told MTV.com: "Michael Jackson was extraordinary. When we worked together on Bad, I was in awe of his absolute mastery of movement on the one hand, and of the music on the other.
"Every step he took was absolutely precise and fluid at the same time. It was like watching quicksilver in motion.
"He was wonderful to work with, an absolute professional at all times, and - it really goes without saying - a true artist. It will be a while before I can get used to the idea that he's no longer with us."
Spielberg told Entertainment Weekly: "Just as there will never be another Fred Astaire or Chuck Berry or Elvis Presley, there will never be anyone comparable to Michael Jackson.
"His talent, his wonderment and his mystery make him legend."
Demi Moore paid tribute on Twitter, writing: "I am greatly saddened for the loss of both Farrah Fawcett and Michael Jackson. Especially for their children!"
Her husband, Ashton Kutcher, also posted: "RIP. Sending love and light to family and friend but especially his kids."
The Rev Al Sharpton, a leading US civil rights campaigner and friend of Jackson, told Sky News: "As a friend of Michael's for the past 35 years, I call on people from around the world to pray for him and his family."
Singer Cher said on the BBC website: "I'm having a million different reactions I didn't expect I would feel.
"He was a great singer - God gives you certain gifts and this child was just an extraordinary child touched by this ability. He could sing like nobody else and he was able to connect with people."
Fellow singer Celine Dion added: "I am shocked. I am overwhelmed by this tragedy. Michael Jackson has been an idol for me all my life.
"He was not only a talented person but he was unique - a genius. It's such a loss. It feels like when Kennedy died, when Elvis died."
Friend and actor Joe Pesci told Fox News: "He was a fun kid and did so much. He always reinvented himself time and time again and certainly was a great entertainer and dancer and great to watch with all that energy.
"He was excited about getting back to the stage. He was downhearted for a while and I think he finally got himself up."
In a tribute reported on the website MTV.com, singer Justin Timberlake said: "I can't find the words right now to express how deeply saddened I am by Michael's passing.
"We have lost a genius and a true ambassador of not only pop music, but of all music.
"He has been an inspiration to multiple generations, and I will always cherish the moments I shared with him on stage and all of the things I learned about music from him and the time we spent together. My heart goes out to his family and loved ones."
And a tribute on the star's website from his record company described him as a genius.
The head of the Sony Corporation Sir Howard Stringer said: "Michael Jackson was a brilliant troubadour for his generation, a genius whose music reflected the passion and creativity of an era. His artistry and magnetism changed the music landscape forever.
"We have been profoundly affected by his originality, creativity and amazing body of work. The entire Sony family extends our deepest condolences to his family and to the millions of fans around the world who loved him."
Broadcaster Paul Gambaccini paid tribute to the "biggest world star since the Beatles" and said Jackson would really be remembered for his career as a solo artist.
Speaking to the BBC, he said: "In decades to come, people will look back and remember the historic records."
He insisted "the freak show" part of Jackson's life would fade into the background.
He added: "You have to recognise that he was the first American male star of the video era - Madonna was the first female star.
"It was Michael Jackson and Madonna who were the first big stars of the MTV generation."
Celine Dion later told CNN presenter Larry King: "I'm overwhelmed by this tragedy. I have to say to you that Michael Jackson's been an idol for me all my life.
"I remember being in in my house when I was very, very young and having his posters above my bed.
"And my goal was to be maybe doing the same showbusiness world as him. And I was listening to his music. And I hoped to be meeting him one day. And I have to tell you that I was very privileged to be on the same record company as him.
"He was so kind. He gave me his autograph. And I have a hat of his with his autograph. And it always has an amazing meaning to me. But now, it has a totally different meaning to me."
She added: "Michael Jackson was not only an extremely talented person, he was unique and he was a genius. It feels like when Kennedy died, when Elvis Presley died.
"I think Michael Jackson lived under pressure all the time, since he was five years old - wanting to please his family and his fans and putting the bar so high that he needed to be suppressing his own self."
Cher told King: "He was a great singer. You know, it's like God gives you certain gifts. And some people he gives different gifts and some people he gives more gifts.
"And this child was just an extraordinary child, touched by this ability to have people feel him and feel people. You know, he was a genius, like Ray Charles, like Stevie Wonder. They just have this gift.
"You don't know where it comes. You don't know how long it stays. You don't know what the impact is going to be."
She added: "When I think of him, I think of this young boy - that teenager that I first met; this, like, adorable boy that I met who, you know, loved to look at my beaded socks and said to me 'Cher, do you think we could just go to the movies?'.
"And we looked at each other and we went 'nah, I don't think so'.
"I think of him more like that, because in later years, I didn't see him that much and when I did see him, his behaviour was very strange to me. I didn't relate to that person as well. I didn't really understand him."
Rock singer Alice Cooper said: "Michael Jackson was easily as influential as James Brown, and that's saying a lot. He was the Fred Astaire of his time.
"We had Vincent Price in common - I used him first on Welcome To My Nightmare in 1975, and he later used him on Thriller. Nobody moved like Michael, he was truly the King of Pop."
Downing Street said the singer's death was "very sad news" for his fans and that Gordon Brown's thoughts were with his family.
"This is very sad news for the millions of Michael Jackson fans in Britain and around the world," the Prime Minister's spokesman said.
"The Prime Minister's thoughts are with Michael Jackson's family at this time."
Radio 2 presenter Chris Evans said the Jackson he had met was a slick professional, unlike the usual caricatures.
He encountered the singer backstage at rehearsals for the Brit Awards. "What I saw was not some goofball with a hankie tied around his mouth, but a guy in jeans and a green t-shirt totally at ease with himself and all those around him, busy putting the kids through their paces of a complicated dance routine.
"He led by example and made them gasp open-mouthed with the way he moved whenever he showed them how to do something, laughing and joking with them to get his point across like all the best teachers.
"When Jacko came on to accept his 'special award' he did his usual squeaky 'I'm-so-shy-I-don't-know-what-to-do' act, presumably because he thought this is what we wanted. If only he felt the world could have seen him as he was the evening before ... a real cool dude getting on with his own genius."
Chart star Lenny Kravitz said: "He was a boy that God blessed with the most angelic voice. Just listen to an early Jackson Five album and listen to that voice. The purity, the feeling, the interpretation of the lyrics.
"At the age of 10, he was on par with James Brown and Aretha Franklin. But the thing he had that they could not match was the fact that he was a child. But at the same time he sang as an adult. That line between child and experienced adult took his expression to an unmatched level.
"I got to work with Michael on a track that has not been released and it was the most amazing experience I've had in the studio. He was funny. Very funny and we laughed the whole time. I also saw what a beautiful father he was. He was a beautiful human being."
Kravitz added: "If not for him, I wouldn't be doing what I'm doing. He gave me joy as a child and showed me the way to go."
Singer Mariah Carey said: "I am heartbroken. Let us remember him for his unparalleled contribution to the world of music, his generosity of spirit in his quest to heal the world, and the joy he brought to his millions of devoted fans throughout the world.
"I feel blessed to have performed with him several times and to call him my friend. No artist will ever take his place. His star will shine forever."
A tribute on Coldplay's Twitter page said: "MJ was the best of the best. His music and performances made the world a brighter place. His light will shine on forever."
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