LOS ANGELES — Michael Jackson, a seminal figure in music, dance and culture whose ever-changing face graced the covers of albums that sold more than half a billion copies, died yesterday, shortly after going into cardiac arrest at his Holmby Hills chateau. He was 50 years old.

Jackson spent much of his life as among the world's most famous people, and to many, his death felt unthinkable and, oddly, inevitable.

"I can't stop crying over the sad news," Madonna said in a statement. "The world has lost one of the greats, but his music will live on forever."

Said Dick Clark: "Of all the thousands of entertainers I have worked with, Michael was the most outstanding. Many have tried and will try to copy him, but his talent will never be matched."

Paramedics found Jackson in cardiac arrest when they arrived at his home shortly before 12:30 p.m., three minutes and 17 seconds after receiving a 911 call. His personal physician already was in the house performing CPR. Jackson was not breathing, and it appears he never regained consciousness.

Paramedics treated Jackson at the house for 42 minutes, and he was declared dead at 2:26 p.m. at the University of California, Los Angeles Medical Center, about two miles from his home.

"It is believed he suffered cardiac arrest in his home. However, the cause of his death is unknown until results of the autopsy are known," his brother Jermaine said.

Police said they were investigating, standard procedure in high-profile cases. Among the factors investigators said they would examine is any medication Jackson might have been taking; an autopsy will be performed today.

Cardiac arrest is an abnormal heart rhythm that stops the heart from pumping blood to the body. It can occur after a heart attack or be caused by other heart problems.

As word of his death spread around the world, MTV switched its programming to play videos from Jackson's heyday. Radio stations began playing marathons of his hits. Hundreds of people gathered outside the hospital. In New York's Times Square, a low groan went up in the crowd when a screen flashed that Jackson had died, and people began relaying the news to friends by cell phone.

"No joke. King of Pop is no more. Wow," Michael Harris, 36, of New York City, read from a text message a friend had sent him. "It's like when Kennedy was assassinated. I will always remember being in Times Square when Michael Jackson died."

Quincy Jones, who produced Jackson's legendary album "Thriller," said he was devastated at the unexpected news.

"For Michael to be taken away from us so suddenly at such a young age, I just don't have the words," he said. "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."

In Sydney, where Jackson married second wife Debbie Rowe in 1996, a celebrity publicist who was a guest at Jackson's Sydney wedding and worked on his Australian tour that year described him as a "tortured genius."

"He was very gentle, very quiet, very shy," Di Rolle told Sky News television. "He was a very complicated, strange man, women loved him and men loved him, too. It's such a sad day, a very sad day."

In the Malaysian capital, Kuala Lumpur — where Jackson's 1996 "HIStory" concert was nearly banned for being too raunchy for the conservative Islamic nation — fans celebrated his influence.

"Hopefully he will always be remembered like Princess Diana," said Noh Yusof, 29, a legal adviser.

In central Mexico City, Esteban Rubio, 30, organized an impromptu homage. Rubio has spent half his life as a Jackson impersonator.

"Respectfully, lovingly, I was preparing a show based on him," he told The Associated Press. "I feel sad, as if a part of my life were torn away."

COMEBACK TOUR

Jackson — who most famously resided in the Santa Ynez Valley at his Neverland Valley Ranch, named for the island where Peter Pan and the Lost Boys were in no danger of growing up — had taken up residence for the rehearsals in a seven-bedroom estate near Bel Air, which he rented for $100,000 a month.

He had come to town to rehearse for 50 sold-out concerts in London, a run of shows scheduled to kick off July 13 that had been dubbed "This Is It." The concerts were to have been the start of an ambitious career resuscitation designed to begin wiping out Jackson's staggering debt — he was at least $400 million in debt and would have earned $1 million a night — and return the singer to cultural relevancy.

Jackson's backers, concert promoter AEG Live and financier Tom Barrack of Colony Capital envisioned the London appearances as an audition of sorts for a reboot that would go on to include a world tour, movies, a Graceland-like museum, new music and revues in Macau and Las Vegas.

Those close to Jackson have said he had been working diligently to get in shape for his comeback. A year ago, he was gaunt and used a wheelchair, but recently he had been exercising with a trainer in addition to daylong rehearsals with dancers half his age. "He's in great shape," his manager, Frank DiLeo, said last month.

TABLOID GOSSIP

Some of the salacious rumors that circled around Jackson were not true. It does not appear that the singer, contrary to popular myth, bought the Elephant Man's bones. Nor, he insisted, did he bleach his skin; Jackson was diagnosed in the 1980s with vitiligo and lupus, requiring treatments that made his skin — along with the pancake makeup he applied to just go to the mall — increasingly pale.

But the tabloids weren't all wrong.

Over the years, interviews, court papers and documentaries revealed a barrage of strange and destructive behavior. Jackson reportedly became addicted to painkillers, sedatives and more, and he was forced into drug rehab. There was a failed marriage, to Lisa Marie Presley, Elvis' daughter. Plastic surgery and severe weight loss altered his appearance; his nose, in particular, seemed in danger of disappearing altogether.

Then, in 2003, after the release of a documentary in which Jackson was seen holding hands and arranging sleepovers with a teenage boy — and after years of murmurs that Jackson had cultivated unusual relationships with children — Jackson was charged with seven counts of child sexual abuse.

His fans and defenders — among them Elizabeth Taylor — argued that Jackson was, in effect, still a child. Indeed, during Jackson's five-month trial in Santa Maria, one doctor determined that Jackson had become a regressed 10-year-old boy and was not a pedophile. In May 2005, he was acquitted on all counts, but his career had never recovered.

"Michael lived a tortured life," said Tommy Mottola, who oversaw Jackson's career for 16 years as a head of Sony Music. "With his successes came all the pressures. ... Imagine living with that stress on a minute-to-minute, day-by-day basis. And that's going on in life from the age of 5 or 6 to 50. It's almost shocking he made it through this long."

Jackson's comeback, Mottola said, represented "a very important platform for him to bridge the old and new." But Jackson also needed the comeback to reverse the damage done by years of excessive spending and little work. He had not toured since 1997 or released a new album since 2001, but continued to live like a megastar.

MEGASTAR SPENDING

To finance his opulent lifestyle, he borrowed heavily against his three main assets: his Neverland Ranch, his music catalog and a second catalog that includes the music of the Beatles co-owned with Sony Corp.

By the time of his criminal trial, he was spending $30 million more annually than he was taking in. Compounding his money difficulties were a revolving door of litigious advisers and hangers-on. Jackson had run through 11 managers since 1990, according to DiLeo.

AEG spent more than $20 million to mount the London shows, flying in dancers to audition and hiring top-notch talent to facilitate the singer's comeback. Kenny Ortega, the force behind "High School Musical" and "Dirty Dancing," was brought on as the concerts' director.

Mottola suggested that the demands of preparing for the tour might have been too much.