The singer was rushed to UCLA Medical Centre earlier tonight after suffering a heart attack.
According to local reports, Jackson was dead when paramedics arrived at his Holby Hills home, while attempts to resuscitate him were "completely unresponsive".
A spokesperson for the Los Angeles City Fire Department said: "The call came in because a person was not breathing.
"When the team arrived, they saw that CPR was already in progress by someone at the home. The person not breathing was transported to UCLA Medical Centre and remains there."
The star's mother and sister LaToya are reportedly inside the hospital, where large crowds have gathered.
He was expected to begin a series of comeback shows at London's O2 arena from July 13.
Jackson is survived by three children, Michael Joseph Jackson, Jr., Paris Michael Katherine Jackson and Prince Michael Jackson II.
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