Paramedics were called to the singer's Beverly Hills home at about midday on Thursday after he stopped breathing.
He was pronounced dead two hours later at the UCLA medical centre. Jackson's brother, Jermaine, said he was believed to have suffered a cardiac arrest.
Speaking on behalf of the Jackson family, Jermaine said doctors had tried to resuscitate the star for more than an hour without success.
He added: "The family request that the media please respect our privacy during this tough time."
"And Allah be with you Michael, always. I love you."
Jackson, who had a history of health problems, had been due to stage 50 concerts at the O2 arena in London, beginning on 13 July.
Concerns were raised last month when four of Jackson's planned comeback shows were postponed, but organisers insisted the dates had been moved due to the complexity of staging the show.
Broadcaster Paul Gambaccini said: "I always doubted that he would have been able to go through that schedule, those concerts. It seemed to be too much of a demand on the unhealthy body of a 50 year old.
"I'm wondering, as we find out details of his death, if perhaps the stress of preparing for those dates was a factor in his collapse.
"It was wishful thinking that at this stage of his life he could be Michael Jackson again."
AT THE SCENE
Rajesh Mirchandani BBC News
Michael Jackson was brought here to the UCLA Medical Center in Los Angeles around 12 hours ago. Earlier there were several hundred people here, before it got dark - there was a sense of grief, of disbelief.
But in the last few hours, these people have been singing his songs, dancing, there was a guy on a keyboard earlier, playing his songs for people to dance along to.
This has turned into an impromptu celebration of Michael Jackson's music. He's the King of Pop as far as they're concerned. They're still shocked by his sudden death, but they're here because they want to show their support.
Following the news of his death, the singer's albums are currently occupying the top 15 slots of online music retailer Amazon.com's best-seller chart, led by his 1982 smash hit Thriller.
Bands playing at the Glastonbury Festival this weekend are also expected to pay tribute to Jackson's musical achievements.
Thriller Live, a tribute show to featuring the star's music in London's West End, announced it would go ahead.
A minute's silence will be held before the performance and the lights outside the Lyric Theatre, in Shaftesbury Avenue, will be dimmed as a mark of respect.
Jackson began his career as a child in family group The Jackson 5.
He went on to achieve global fame as a solo artist with smash hits such as Billie Jean and Bad.
Thriller, released in 1982, is the biggest-selling album of all time, shifting 65m copies, according to the Guinness Book of World Records.
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