He was not breathing when they arrived. The paramedics performed CPR but he was reportedly totally unresponsive.
Fans are arriving outside the hospital.
Close friend Uri Geller told Sky News he was hoping desperately that the reports 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."
"I'm absolutely devestated and shocked. He was a young man terribly fit and basically in good shape. If it is true I am just in total shock."
Joe Jackson, the star's father, told showbiz website TMZ.com: "He is not doing well."
The site reported that the emergency 911 call came in at 12.21pm (8.21pm BST) at his Holmby Hills home in Los Angeles.
A spokesman for UCLA said that due to patient confidentiality the medical centre could not comment on or confirm the reports.
Jackson suffered a cardiac arrest, according TMZ.
Once at the hospital, staff tried to resuscitate him but he was completely unresponsive, the website added.
It said his sister LaToya Jackson ran into the hospital sobbing.
The singer was given cardiopulmonary resuscitation and was not breathing when paramedics arrived at his home, according to the Los Angeles Times. He was taken to the University of California, Los Angeles Medical Centre, the newspaper added.
Jackson was due to perform 50 farewell concerts, bowing out on an illustrious music career spanning several decades which earned him a fortune.
A spokeswoman for The Outside Organisation, who are organising the publicity for Jackson's UK concerts, said she had no comment at this time.
Tonight bookmakers William Hill suspended betting on how many gigs Jackson will do at the O2 in London.
Captain Steve Ruda, of Los Angeles Fire Department, told the LA Times that paramedics responded to a call at Jackson's home around 12.26pm.
Fears were sparked for Jackson's health last month when four of his This Is It concert dates were pushed back, but organisers insisted the dates were moved due to the sheer magnitude of the spectacle.
The first scheduled show was due to take place on July 8 and was pushed back to July 13. But the other three dates were rescheduled to spring 2010.
The London shows would have marked Jackson's first live performance for more than two years.
At the World Music Awards in London in November 2006 he performed just a few lines from We Are The World.
Jackson last toured 12 years ago when he played 82 shows in 58 cities for the HIStory tour.
He has spent time living outside the United States since his 2005 acquittal on child molestation charges.
Jackson has three children, Michael Joseph Jackson Junior, Paris Michael Katherine Jackson and Prince "Blanket" Michael Jackson II.
A cardiologist at UCLA told TMZ.com that Jackson died of cardiac arrest.
Fans have flooded websites to express their grief at the reported death of Jackson.
ElToroDiablo posted on gossip website TMZ.com: "God bless you Michael, heaven just got another beautiful angel."
Kimbers wrote: "What a sad day. I hope Michael Jackson finds the peace that eluded him in life."
Another fan, Sineen, wrote on website MJSite.com: "I never thought that one day would come and i'll have to write this down, Michael Jackson is no longer with us. May your warm soul rest in peace. I'll always love you Michael. Keep the faith!"
Rulintheroost posted on political website The Huffington Post: "RIP Michael. Your music was a joy to many people around the world."
Brian Oxman, a Jackson family lawyer, told CNN he was told by brother Randy Jackson that star collapsed at his home in west Los Angeles this morning.
Matt Blank from the Michael Jackson World Network fanclub told BBC News 24 he was "a bit dumfounded and a bit shellshocked by it all".
He added: "I find it hard to believe this could be a reality.
"In the past, health issues of Michael Jackson have always been rumoured. I can only imagine this is down to the amount of stress he was under to come out and perform."
He added: "It's just unimaginable that Michael Jackson is no more, it's just unbelievable."