Singer Michael Jackson died last night after collapsing at his home in Los Angeles.
The singer stopped breathing and suffered a massive cardiac arrest, although the exact circumstances of his death are unknown.
Paramedics failed to revive the 50 year-old and he was taken to hospital in a coma where he was pronounced dead.
There were unconfirmed reports this morning that Jackson may have suffered a heart attack after receiving an injection of the painkiller Demerol form his private physician.
His mother, brothers, including Jermaine, Tito and Randy and sisters Janet and LaToya, had raced to be at his bedside.
Devastated: Michael Jackson's sister La Toya (left) is seen arriving at the hospital sobbing, followed by his mother, Katherine, on whom he had come to rely in recent months
Elizabeth Taylor, one of Michael Jackson's closest friends, had been planning to fly out on a private jet to attend Jackson's opening night in London, Larry King said on CNN last night.
Jackson's brother, Jermaine Jackson, told reporters during a press conference that he believed Michael had died from a cardiac arrest, but added that the exact cause of death was not known.
Investigators are likely to focus on claims the singer suffered a long-term addiction to painkillers.
Last month it was alleged he was struggling to make even a handful of the rehearsals for his comeback tour in Britain.
It later emerged he had been to just two out of more than 45 rehearsals.
One Jackson family source said at the time he was 'just not able to pick himself up out of the painkiller stupor' that he was in.
Jackson, who has three young children, was seen being taken into an ambulance by Los Angeles Fire Department paramedics and moved to the UCLA Medical Centre.
The Los Angeles Times reported that the star was not breathing when paramedics arrived at his home.
When emergency staff arrived, they began resuscitation procedures immediately.
Ten bodyguards followed the ambulance to hospital where the star was rushed into the emergency room on a stretcher.
Doctors again tried to revive him, but attempts failed.
Jermaine Jackson told reporters that medical staff had tried to resuscitate his brother for an hour after he collapsed at his home.
"His personal physician, who was with him at the time, attempted to resuscitate my brother, as did the paramedics who transported him to the UCLA," he said.
"On arrival at 1.14pm (local time) a team of doctors, including emergency physicians and cardiologists, attempted to resuscitate him for one hour but they were unsuccessful."
Jackson's mother and brothers raced from the family home nearby to be at his bedside.
A source close to his family said: 'It's so tragic but he was such a wreck physically that all the signs were there.'
His ex-wife Lisa Marie Presley said that she was 'heartbroken' for his children.
'I am so very sad and confused with every emotion possible. Words fail me,' she said in a statement.
His sister La Toya was spotted running in the UCLA Medical Centre sobbing shortly after he was pronounced dead.
Katherine Jackson, Michael's mother, arrived at UCLA hospital to be with the rest of the family shortly after 3pm.
He said: 'As someone who served as Michael Jackson's publicist during the firstst child molestation incident, I must confess I am not surprised by today's tragic news.
'Michael has been on an impossibly difficult and often self-destructive journey for years. His talent was unquestionable but so too was his discomfort with the norms of the world.'
Jackson had apparently been 'doped up to his eyes' for several weeks with anti-pain prescriptions including Vicodin.
The source, who asked not to be named, said: 'He has pain in his back and that is the reason for the drugs, but it is obvious that the painkillers have taken over his life.'
Another family friend said that his ex-wife Debbie Rowe is expected to receive initial custody of the children while long-term arrangements are sorted.
Another family source said he had become increasingly withdrawn over recent weeks, leaning on his mother Katherine for support as his health deteriorated.
'He had been so frail, both physically and emotionally,' the acquaintance said. 'Many warned he was pushing too hard and overdoing things, but he wanted to get back to work so badly, he kept pushing himself harder and harder.
'He didn't want to let down his fans, and he desperately needed the cash.'
Tributes poured in immediately from the world of celebrity.
Publicist Max Clifford first met the star in the 60s when Mr Clifford was working at record label EMI's press office. He said he had spoken to the singer most recently three weeks before reality TV star Jade Goody died earlier this year when Jackson asked to visit her.
Mr Clifford said: "It is a terrible shock, you wonder if the strain of getting fit for this major tour proved too much. In recent pictures he looked anything but healthy. That's the first thought that went through my mind.
'He was always someone who seemed to find it difficult to cope with fame. He was a great talent. He was already a superstar when he was in his teens but then he was with four others. Then all of a sudden he was on his own and he did seem to find it difficult to cope.'
Music producer Quincy Jones said: 'I am absolutely devastated at this tragic and unexpected news.'
He said Jackson 'had it all - talent, grace, professionalism and dedication'.
He added: 'I've lost my little brother today, and part of my soul has gone with him.'
Twitter was alive with celebrity tributes. Actress Demi Moore wrote: 'I am greatly saddened for the loss of both Farrah Fawcett and Michael Jackson. Especially for their children!'Tributes:
Support: A mother, Yoshika Plair (right) hugs her daughter Summer outside UCLA as they take in the news of Michael Jackson's death
He added: 'Plea to the press to respect his wishes of maintaining the anonymity of his children. Plea to the public to refuse to consume media that does not respect the anonymity of Michael's children.'
Foreign Secretary David Miliband wrote: 'Never has one soared so high and yet dived so low. RIP Michael.'
Peter Andre wrote: 'Michael Jackson dying is absolutely devastating. I am totally shocked. MJ, you're the best.'
Close friend Uri Geller initially told Sky News he was hoping desperately that the reports of Jackson's death were not true.
Jackson's friend Mark Lester, the child star from the 1968 film Oliver! who is also godfather to the singer’s children said: 'He was the most wonderful, lovely guy in the world. I can't believe what's happened.
'I don't know what to say. It's just so horrible.’
The star had three children, the elder two, Prince Michael I, 12, and Paris Michael, 11, with ex-wife Debbie Rowe. The mother of the youngest child, seven-year-old Prince Michael II, has never been revealed.
There had been persistent rumours about Jackson's failing health, most recently that he was suffering from skin cancer.
He was frequently pictured looking frail, and in recent years had often been photographed in a wheelchair or using a walking stick.
In May he spent three days in conference with doctors at the Bedford Medical building in Los Angeles.
Those following the electronic newsboard react with shock
Jackson subsequently denied these allegations but failed to shake off fears about his wellbeing.
The troubled star, who had numerous cosmetic surgery procedures to alter his appearance, was also diagnosed with the dermatologic disorder Vitiligo, which causes depigmentation in patches of skin, which he claimed to be the reason for white skin.
He also suffered Lupus – a tissue disease.
Earlier this year, the 50-year-old's fans were left worried and disappointed after he cancelled the opening four shows of his sell-out tour. Despite the reported fears for his health and fitness, his representatives had previously insisted he would be able to perform.
More than 750,000 tickets have been sold for the 'This is It' concerts, which would have earned him as much as £50million, enough to clear his debts.
Fans who have already bought tickets for his UK dates will now be concerned about getting refunds for the series of shows, including a number of appearances at the O2.
Despite being one of the most successful recording artists of all time with hits such as Thriller, Bad and Billie Jean, he had amassed massive debts.
His former home - the famous Neverland Ranch - has been taken over by developers Floral tribute: A young fan lays a flower in memory of Michael Jackson outside the singer's former childhood home
Jackson, who had not performed since the 1990s, saw his career and popularity plummet after a child abuse scandal four years ago.
Despite being cleared by a Californian court over allegations that he had abused young boys on sleep-overs at his home he never recovered from the negative press attention.
He left the Neverland Ranch and after stays in Bahrain and Europe returned to the U.S. where he faced near bankruptcy, being described as a 'millionaire who spends like a billionaire'.