Michael Jackson, one of pop music's biggest stars, died on Thursday at a Los Angeles hospital after having suffered a cardiac arrest. He was 50.
The cause of death is not known and a post-mortem examination is likely to take place on Friday, Los Angeles coroner Fred Corral said.
Jackson was taken ill at Beverley Hills home and his personal physician tried to resuscitate him but could not, his brother Jermaine told reporters.
The singer was taken by paramedics to UCLA hospital, where doctors worked for more than an hour to try to revive him, before pronouncing him dead at 2.26pm.
Jackson had a history of health problems and had been due to start a series of comeback concerts in London on 13 July running until March 2010. The singer had been rehearsing for the tour just hours before his death.
A lawyer for the Jackson family said the singer was taking prescription drugs as he battled to get back into shape for the gruelling comeback.
Brian Oxman says Jackson, who fought long-running battles with prescription medication throughout his 45-year career, was taking the drugs after suffering injuries during training for the tour.
Mr Oxman told CNN that the family had been concerned about Jackson's health and had tried in vain to take care of him for months.
The promoters of the London shows, AEG Live, said in March that Jackson had passed a four-and-a-half hour physical examination with independent doctors.
He leaves behind a mountain of debt. Many hoped the shows would rake in millions and erase his financial troubles.
Millions are mourning his death worldwide, while in Los Angeles hundreds have gathered outside UCLA hospital.
The child star turned King of Pop set the world dancing, but his musical genius was overshadowed by a bizarre lifestyle and sex scandals.
Jackson's hits included Thriller and Billie Jean and his dramatic, one-gloved stage presence and innovative dance moves were imitated by legions of fans around the world.
His lifetime record sales tally is believed to be about 750 million, which, added to the 13 Grammy Awards he received, made him one of the most successful entertainers of all time.
But his preference for the company of children, his high-pitched voice and numerous plastic surgeries also earned him critics and the nickname "Wacko Jacko."
Jackson lived as a virtual recluse following his 2005 acquittal on charges including child molestation and plotting to kidnap his young accuser.
In 1994, Jackson married Lisa Marie Presley, the 26-year-old daughter of Elvis Presley. The marriage lasted less than two years.
He then married Debbie Rowe in Sydney, a 37-year-old nurse he met while undergoing plastic surgery in 1997. They had two children, Prince Michael and Paris Michael Katherine, before divorcing in 1999.
Jackson had custody of the two children and of a third, Prince Michael II, born to an unknown woman.
Fame first with Jackson Five
Born on August 29, 1958, Jackson made his show business debut with four of his older brothers in the Jackson Five pop group, and went on to lead the stage clan with a piping soprano and dazzling dance moves.
By 1969, the group had signed a contract with Motown Records, becoming one of the last great acts to emerge from the legendary label.
The Jacksons produced seven platinum singles for Motown, selling over a million, and three multi-platinum albums, selling more than two million. They moved to CBS's Epic Records in 1976.
Despite the early success, Jackson was to recall those years as unhappy and lonely ones. Eventually the family act broke up, as Jackson went solo.
In 1979, Quincy Jones produced Jackson's first solo album for Epic, Off the Wall, a huge disco-oriented success that sold 10 million copies.
They teamed up again in 1982 for what would be Jackson's breakthrough album as a composer and co-producer, Thriller, which became the top-selling album of all time, selling 65 million copies, according to the Guinness Book of World Records.
Tributes for 'consummate entertainer'
Tributes have poured in worldwide following news of Jackson's sudden death.
Lisa Marie Presley said in a statement: "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."
Music producer Quincy Jones said: "I am absolutely devastated at this tragic and unexpected news. For Michael to be taken away from us so suddenly at this young age, I just don't have the words."
"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."
The father of Motown Records, Berry Gordy, said: "Michael Jackson doesn't just come along once in a century or a lifetime; he only comes along once. And so we had the benefit of enjoying him while he was here and we will enjoy him forever through his music."
Pop star Madonna said in a statement: "I can't stop crying over the sad news. ... I have always admired Michael Jackson. The world has lost one of the greats but his music will live on forever."
New Zealand singer Annie Crummer, who supported Jackson's shows in Auckland in 1996, told Radio New Zealand that she was shocked by the news. She said Jackson had a huge influence on her career and musical path.
Filmmaker Steven Spielberg told Entertainment Weekly magazine: "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."