LOS ANGELES — Michael Jackson, the "King of Pop" who once moonwalked above the music world, died Thursday as he prepared for a comeback bid to vanquish nightmare years of sexual scandal and financial calamity. He was 50.

Jackson died at UCLA Medical Center after being stricken at his rented home in Holmby Hills. Paramedics tried to resuscitate him at his home for nearly three-quarters of an hour, then rushed him to the hospital, where doctors continued to work on him.

"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.

Jackson's death brought a tragic end to a long, bizarre, sometimes farcical decline from his peak in the 1980s, when he was popular music's premier all-around performer, a uniter of black and white music who shattered the race barrier on MTV, dominated the charts and dazzled even more on stage.

His 1982 album "Thriller" — which included the blockbuster hits "Beat It," "Billie Jean" and "Thriller" — is the best-selling album of all time, with an estimated 50 million copies sold worldwide.

At the time of his death, Jackson was rehearsing hard for what was to be his greatest comeback: He was scheduled for an unprecedented 50 shows at a London arena, with the first set for July 13.

As word of his death spread, 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."

In his hometown of Gary, Ind., fans formed a prayer circle on the front yard of Jackson's modest childhood home and hundreds more crowded the street. Stuffed teddy bears and roses were left on the porch of the small white frame house, which now sits empty.

The public first knew him as a boy in the late 1960s, when he was the precocious, spinning lead singer of the Jackson 5, the singing group he formed with his four older brothers. Among their No. 1 hits were "I Want You Back," "ABC" and "I'll Be There."