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.
His mother, brothers including Jermaine, Tito and Randy and sisters Janet and LaToya, had raced to be at his bedside.
He had been very frail in recent weeks as he struggled to prepare for a 50 concerts in London, which were due to start on July 13.
Elizabeth Taylor, one of Michael Jackson's closest friends, had planned 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.
Police said that an emergency call was made from the house at 12.21pm local time [8.21pm uk] today and paramedics raced to the scene.
Comeback tour: The singer was due to start a land-mark tour at the 02 in London next month
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, according to celebrity website TMZ.
The Los Angeles Times reported that the star was not breathing when paramedics arrived at his home.
When emergency staff arrived, they were forced to begin 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.
His sister, La Toya, was seen running into the hospital sobbing.
A source close to his family said tonight: '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.
Jackson's longtime publicist Michael Levine said he wasn't surprised by the news.
He said: "As someone who served as Michael Jackson's publicist during the 1st 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.'
He added: 'His family is telling him to ditch the medications and get on with the tour.'
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.
As news broke of Jackson's heart attack, a member of the Jackson family told TMZ that 'Michael is in really bad shape'.
The website also spoke to Joe Jackson, Michael's dad, who said "he is not doing well".'
Tributes poured in last night 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 spoke 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.
Her husband, Ashton Kutcher, also posted: "RIP. Sending love and light to family and friend but especially his kids."
He added: "Plea to the press to respect his wishes of maintaining the anonymity of his children. I plea to the public to refuse to consume media that does not respect the anonymity of Michaels children."
Foreign Secretary David Miliband wrote on Twitter: "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 told Sky News he was hoping desperately that the reports of Jackson's death were not true.
"I must hear it from a doctor. I cannot believe everything I see and read and hear at the moment. I hope its not true, I'm waiting like you are, like the whole planet is waiting to hear it from the mouth of the doctor taking car of him," he said.
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 wondeful, lovely guy in the world. I can't believe what's happened.
'I don't know what to say. It's just so horrible.’
Hundreds of fans have started to gather outside the hospital, hoping for news of the King of Pop.
The star had three children, the elder two, Prince Michael I, 12, and Paris Michael, 11, with ex-wife Debbie Rowe.
It has never been revealed who the mother is of the star’s youngest child, seven-year-old Prince Michael II. There had been persistent rumours about his 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.
His biographer claimed the troubled star was suffering from emphysema and chronic gastrointestinal bleeding, which his doctors had trouble stopping.
Jackson subsequently denied these allegations but failed to shake off fears about his well being.
The troubled star, who has numerous cosmetic surgery procedures to alter his appearance, also battled an addiction to prescription drugs. He was 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 earlier this year.
Despite 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 former King of Pop's "This is It "concerts.
Jackson had needed to perform at the concerts, which would have earned him as much as £50m, to clear his debts.
Despite being one of the most successful recording artists of all time with hits such as Thriller, Bad and Billie Jean, he amassed massive debts.
His former home - the famous Neverland Ranch has been taken over by developers.
The singer was also forced to settle a £3m lawsuit with a Bahrainian prince over his failure to write a book and perform in a concert in return for a year's free stay in Bahrain.
Jackson, who has 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 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 US where he faced near bankruptcy.
Jackson was described as a "millionaire who spends like a billionaire".
The cash-strapped star was also reduced to selling treasures from his Neverland ranch.