For his legions of fans, he was the Peter Pan of pop music: the little boy who refused to grow up. But on the verge of another attempted comeback, he is suddenly gone, this time for good.

Michael Jackson, whose quintessentially American tale of celebrity and excess took him from musical boy wonder to global pop superstar to sad figure haunted by lawsuits and failed plastic surgery, was pronounced dead Thursday afternoon at the Ronald ReaganUCLA Medical Center. Mr. Jackson was just 50 years old, having spent 39 of those years in the public eye he loved.

The singer was taken to the hospital, a six-minute drive from the rented Bel-Air home in which he was living, shortly after noon by paramedics for the Los Angeles Fire Department.

Jermaine Jackson said his brother was pronounced dead at 2:26 p.m. “It is believed he suffered cardiac arrest at his home,” he said softly at a news conference at the hospital. A personal physician first attempted to resuscitate Michael Jackson before paramedics arrived. A team of doctors attempted to resuscitate Mr. Jackson for more than an hour, his brother said. An autopsy will be performed to determine the cause of death.

“May our love be with you always,” Jermaine Jackson concluded his gaze aloft.

As with Elvis Presley or the Beatles, it is impossible to calculate the full effect he had on the world of music. At the height of his career, he was indisputably the biggest star in the world; he has sold more than 750 million albums. Radio stations across the country reacted to his death with marathon sessions of his songs. MTV, which was born in part as a result of Mr. Jackson’s groundbreaking videos, reprised its early days as a music channel by showing his biggest hits.

From his days as the youngest brother in the Jackson 5 to his solo career in the 1980s and early 1990s, Mr. Jackson was responsible for a string of hits like “I Want You Back,” “I’ll Be There” “Don’t Stop ‘Til You Get Enough” “Billie Jean” and “Black and White” that exploited his high voice, infectious energy and ear for irresistible hooks.

As a solo performer, Mr. Jackson ushered in the age of pop as a global product – not to mention an age of spectacle and pop culture celebrity. He became more character than singer: his sequined glove, his whitened face, his moonwalk dance move became embedded in the cultural firmament.

But not long after his entertainment career hit high-water marks – “Thriller,” from 1982, has been certified 28 times platinum by the Recording Industry Association of America – it started a bizarre disintegration. His darkest moment undoubtedly came in 2003, when he was indicted on child molesting charges. A young cancer patient claimed the singer had befriended him and then fondled him at his Neverland estate near Santa Barbara, Calif., but Mr. Jackson was acquitted on all charges.

Reaction started trickling in from the entertainment community late Thursday.

“I am absolutely devastated at this tragic and unexpected news,” the music producer Quincy Jones said in a statement. Mr. Jones, who produced “Thriller” said Mr. Jackson “had it all – talent, grace, professionalism and dedication.” He added, “I’ve lost my little brother today, and part of my soul has gone with him.”

The Apollo Theater, where Mr. Jackson and his brothers first performed in 1969 (they won amateur night), said, “We will always remember Michael in our hearts as a true Apollo legend, known for his professionalism and grace.”

Kenny Ortega, the director and choreographer who was working with Mr. Jackson to create the London concert series, called “This is It,” said, “This is all too much to comprehend.”

Impromptu vigils broke out around the world, from Portland, Ore., where fans organized a one-gloved bike ride (“glittery costumes strongly encouraged”) to Hong Kong, where fans gathered with candles and sang his songs.

Mr. Jackson was an object of fascination for the news media since his first hit, “I Want You Back,” in 1969. His public image wavered between that of the musical naif, who wanted only to recapture his youth by riding on roller-coasters and having sleepovers with his friends, to the calculated mogul who carefully constructed his persona around his often-baffling public behavior.

Jackson married Lisa Marie Presley in 1994, and they divorced in 1996. Later that year, Jackson married Deborah Rowe, a former nurse for his dermatologist. They had two children together: Michael Joseph Jackson Jr., known as Prince Michael, and Paris Michael Katherine Jackson. Rowe filed for divorce in 1999.

He had cemented his place in pop culture. He got the plum Scarecrow role in the 1978 movie musical “The Wiz,” a pop-R&B version of “The Wizard of Oz,” that starred Diana Ross as Dorothy.

During production of a 1984 Pepsi commercial, Jackson’s scalp sustains burns when an explosion sets his hair on fire.

He had strong follow-up albums with 1987′s “Bad” and 1991′s “Dangerous,” but his career began to collapse in 1993 after he was accused of molesting a boy who often stayed at his home. The singer denied any wrongdoing, reached a settlement with the boy’s family, reported to be $20 million, and criminal charges were never filed.

Jackson’s expressed anger over the allegations on the 1995 album “HISTORY,” which sold more than 2.4 million copies, but by then, the popularity of Jackson’s music was clearly waning, even as public fascination with his increasingly erratic behavior was growing.

Mr. Jackson had been scheduled to perform 50 concerts at the O2 arena in London beginning next month and continuing into 2010. The shows were positioned as a comeback, with the potential to earn him up to $50 million, according to some reports.