Michael Jackson's brother Jermaine has revealed the team at UCLA Medical Center tried to resuscitate Jackson for over an hour.
Speaking to media on behalf of the family, Jermaine said his brother suffered a cardiac arrest, and asked for "privacy at this tough time".
"My brother Michael Joseph Jackson passed away on Thursday June 25 at 2.26pm," Jermaine Jackson said.
"His personal physician, who was with him at the time, attempted to resuscitate my brother, as did the paramedics as they transported him to Ronald Reagan Medical Center in Los Angeles.
"May our love be with you Michael, always. We love you."
Friends of Jackson have led tributes to the "King of Pop" as hundreds of fans gather outside the Medical Center.
Reports of the singer's death began to surface soon after he was taken to the hospital around 7.30am NZST.
A short time ago Ed Winter, the assistant chief coroner for Los Angeles County, confirmed his office had been notified of the death and would handle the investigation.
OK! magazine is reporting another person from the coroner's office, Fred Corral, as saying an autopsy will likely be carried out on Friday local time, and that the cause of death may be known as soon as Friday night local time.
"For Michael to be taken away from us so suddenly at such a young age, I just don't have the words," said Quincy Jones, who produced Thriller.
"He was the consummate entertainer and his contributions and legacy will be felt upon the world forever. I've lost my little brother today, and part of my soul has gone with him."
Singer Dionne Warwick said: "Michael was a friend and undoubtedly one of the world's greatest entertainers that I fortunately had the pleasure of working with. ... We have lost an icon in our industry."
A large crowd has gathered outside the hospital and traffic jams are blocking the area while news helicopters whirr overhead.
The emergency entrance at the hospital, which is near Jackson's rented home, was roped off with police tape.
Elsewhere, fans have gathered on the Hollywood Walk of Fame and in New York's Times Square to pay tribute to Jackson.
"Ladies and gentlemen, Michael Jackson has just died," a woman boarding a Manhattan bus called out, shortly after the news was announced. Immediately many riders reached for their cell phones.
So many people wanted to verify the early reports of Jackson's death that the computers running Google's news section interpreted the fusillade of "Michael Jackson" requests as an automated attack for about half an hour.
In Times Square, a low groan went up in the crowd when a screen flashed that Jackson had died, and people began relaying the news to friends by cell phone.
"No joke. King of Pop is no more. Wow," Michael Harris, 36, of New York City, read from a text message a friend sent to his telephone.
"It's like when Kennedy was assassinated. I will always remember being in Times Square when Michael Jackson died."
Jackson was not breathing when Los Angeles Fire Department paramedics responded to a call at his Los Angeles home about 12.30pm local time (7.30am NZST), Captain Steve Ruda told the Los Angeles Times.
The paramedics performed CPR and took him to the hospital, Ruda told the newspaper.
Jackson's death brought a tragic end to a long, bizarre, sometimes farcical decline from his peak in the 1980s, when he was popular music's premier all-around performer, a uniter of black and white music who shattered the race barrier on MTV, dominated the charts and dazzled even more on stage.
His 1982 album Thriller - which included the blockbuster hits Beat It, Billie Jean and "Thriller" - is the best-selling album of all time, with an estimated 50 million copies sold worldwide.
The public first knew him in the late 1960s, when as a boy he was the precocious, spinning lead singer of the Jackson 5, the music group he formed with his four older brothers.
Among their No. 1 hits were I Want You Back, ABC, and I'll Be There.
He was perhaps the most exciting performer of his generation, known for his feverish, crotch-grabbing dance moves and his high-pitched voice punctuated with squeals and titters.
His single sequinned glove, tight, military-style jacket and aviator sunglasses were trademarks second only to his ever-changing, surgically altered appearance.
Jackson ranked alongside Elvis Presley and the Beatles as the biggest pop sensations of all time. He united two of music's biggest names when he was briefly married to Presley's daughter, Lisa Marie, and Jackson's death immediately evoked that of Presley himself, who died at age 42 in 1977.
As years went by, Jackson became an increasingly freakish figure - a middle-aged man-child weirdly out of touch with grown-up life. His skin became lighter, his nose narrower, and he spoke in a breathy, girlish voice.
He surrounded himself with children at his Neverland ranch, often wore a germ mask while travelling and kept a pet chimpanzee named Bubbles as one of his closest companions.
"It seemed to me that his internal essence was at war with the norms of the world. It's as if he was trying to defy gravity," said Michael Levine, a Hollywood publicist who represented Jackson in the early 1990s.
He called Jackson a "disciple of P.T. Barnum" and said the star appeared fragile at the time but was "much more cunning and shrewd about the industry than anyone knew".
Jackson caused a furore in 2002 when he playfully dangled his infant son, Prince Michael II, over a hotel balcony in Berlin while a throng of fans watched from below.
In 2005, he was cleared of charges he molested a 13-year-old cancer survivor at Neverland in 2003. He had been accused of plying the boy with alcohol and groping him, and of engaging in strange and inappropriate behaviour with other children.
The case followed years of rumours about Jackson and young boys. In a television documentary, he had acknowledged sharing his bed with children, a practice he described as sweet and not at all sexual.
Despite the acquittal, the lurid allegations that came out in court took a fearsome toll on his career and image, and he fell into serious financial trouble.
Jackson was preparing for what was to be his greatest comeback: He was scheduled for an unprecedented 50 shows at a London arena, with the first set for July 13.
He was in rehearsals in Los Angeles for the concert, an extravaganza that was to capture the classic Jackson magic: showstopping dance moves, elaborate staging and throbbing dance beats.
- AP, NZ HERALD STAFF
Tributes flow for Michael Jackson
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