Pop star Michael Jackson has died in Los Angeles, aged 50.

Paramedics were called to the singer's Beverly Hills home at about midday on Thursday after he stopped breathing.

He was pronounced dead two hours later at the UCLA medical centre. Jackson's brother, Jermaine, said he was believed to have suffered a cardiac arrest.

Jackson, who had a history of health problems, had been due to stage a series of comeback concerts in the UK on 13 July.

Speaking on behalf of the Jackson family, Jermaine, said doctors had tried to resuscitate the star for more than an hour without success.

He added: "The family request that the media please respect our privacy during this tough time."

"And Allah be with you Michael always. I love you."

Concerns were raised last month when four of Jackson's planned comeback concerts were postponed, but organisers insisted the dates had been moved due to the complexity of staging the show.

A spokeswoman for The Outside Organisation, which was organising the publicity for the shows, said she had no comment at this time.

Broadcaster Paul Gambaccini said: "I always doubted that he would have been able to go through that schedule, those concerts. It seemed to be too much of a demand on the unhealthy body of a 50 year old.

"I'm wondering that, as we find out details of his death, if perhaps the stress of preparing for those dates was a factor in his collapse.

"It was wishful thinking that at this stage of his life he could be Michael Jackson again."

Uri Gellar, a close friend of the star, told BBC News it was "very, very sad".

Speaking outside New York's historic Apollo theatre, civil rights activist Rev Al Sharpton paid tribute to his friend.

"I knew him 35 years. When he had problems he would call me," he said.

"I feel like he was not treated fairly. I hope history will be more kind to him than some of the contemporary media."

Melanie Bromley, west coast bureau chief of Us Weekly magazine, told the BBC the scene in Los Angeles was one of "pandemonium".

"At the moment there is a period of disbelief. There are hundreds of people outside UCLA waiting for news.

"He was buying a home in the Holmby Hills area of Los Angeles and the scene outside the house is one of fans, reporters and TV cameras - it's absolute craziness.

"I feel this is the biggest celebrity story in a long time and has the potential to be the Princess Diana of popular culture."