LOS ANGELES — 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 U.C.L.A. Medical Center after arriving in a coma, a city official said. Mr. Jackson was just 50 years old, having spent 39 of those years in the public eye he loved.

The singer was rushed 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. A hospital spokesman would not confirm reports of cardiac arrest.

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 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 the Jackson 5’s 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.

A Tour Canceled

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.

But there has also been worry and speculation that Mr. Jackson was not physically ready for such an arduous run of concerts, and his postponement of the first of those shows to July 13 from July 8 fueled new rounds of gossip about his health.

“The primary reason for the concerts wasn’t so much that he was wanting to generate money as much as it was that he wanted to perform for his kids,” said J. Randy Taraborrelli, whose biography, “Michael Jackson: The Magic and the Madness,” was first published by Citadel in 1991. “They had never seen him perform before.”

Mr. Jackson’s brothers, Jackie, Tito, Jermaine, Marlon and Randy, have all had performing careers, with varying success, since they stopped performing together. (Randy, the youngest, replaced Jermaine when the Jackson 5 left Motown.) His sisters, Rebbie, La Toya and Janet, are also singers, and Janet Jackson has been a major star in her own right for two decades. They all survive him, as do his parents, Joseph and Katherine Jackson, of Las Vegas, and three children: Michael Joseph Jackson Jr., Paris Michael Katherine Jackson and Prince Michael Jackson II.

Michael Joseph Jackson was born in Gary, Ind., on Aug. 29, 1958. The second youngest of six brothers, he began performing professionally with four of them at the age of 5 in a group that their father, Joe, a steelworker, had organized the previous year. In 1968 the group, originally called the Jackson Brothers, was signed by Motown Records.

The Jackson 5 was an instant phenomenon. The group’s first four singles — “I Want You Back,” “ABC,” “The Love You Save” and “I’ll Be There” — all reached No. 1 on the pop charts in 1970, a feat no group had accomplished before. And young Michael was the center of attention: he handled virtually all the lead vocals, danced with energy and finesse, and displayed a degree of showmanship rare in a performer of any age.

In 1971, Mr. Jackson began recording under his own name, while continuing to perform with his brothers. His recording of “Ben,” the title song from a movie about a boy and his homicidal pet rat, was a No. 1 hit in 1972.

The brothers (minus Michael’s older brother Jermaine, who was married to the daughter of Berry Gordy, Motown’s founder and chief executive) left Motown in 1975 and, rechristened the Jacksons, signed to Epic, a unit of CBS Records. Three years later Michael made his movie debut as the Scarecrow in the screen version of the hit Broadway musical “The Wiz.” But movie stardom proved not to be his destiny.

Music stardom on an unprecedented level, however, was. Mr. Jackson’s first solo album for Epic, “Off the Wall,” released in 1979, yielded four No. 1 singles and sold seven million copies, but it was a mere prologue to what came next. His follow-up, “Thriller,” released in 1982, became the best-selling album of all time and helped usher in the music video age. The video for the album’s title track, directed by John Landis, was an elaborate horror-movie pastiche that was more of a mini-movie than a promotional clip and played a crucial role in making MTV a household name.

Seven of the nine tracks on “Thriller” were released as singles and reached the Top 10. The album spent two years on the Billboard album chart and sold more than 100 million copies worldwide. It also won eight Grammy Awards.

Such accomplishments would have been difficult for anyone to equal, much less surpass. Mr. Jackson’s next album, “Bad,” released in 1987, sold eight million copies and produced five No. 1 singles and another state-of-the-art video, this one directed by Martin Scorsese. It was a huge hit by almost anyone else’s standards, but an inevitable letdown after “Thriller.”

Private Life Issues

It was at this point that Mr. Jackson’s bizarre private life began to overshadow his music. He would go on to release several more albums and, from time to time, to stage elaborate concert tours. And he would never be too far from the public eye. But it would never again be his music that kept him there.

Even with the millions Mr. Jackson earned, his eccentric lifestyle took a severe financial toll. In 1988 Mr. Jackson paid about $17 million for a 2,600-acre ranch in Los Olivos, Calif., 125 miles northwest of Los Angeles. Calling it Neverland after the mythical island of Peter Pan, he outfitted the property with amusement-park rides, a zoo and a 50-seat theater, at a cost of $35 million, according to reports, and the ranch became his sanctum.

But Neverland, and Mr. Jackson’s lifestyle, were expensive to maintain. A forensic accountant who testified at Mr. Jackson’s molesting trial in 2005 said that Mr. Jackson’s annual budget in 1999 included $7.5 million for personal expenses and $5 million to maintain Neverland. By at least the late 1990s, he began to take out huge loans to support himself and pay debts. In 1998 he took out a loan for $140 million from Bank of America, which two years later was increased to $200 million. Further loans of hundreds of millions followed.

The collateral for the loans was Mr. Jackson’s 50 percent share in Sony/ATV Music Publishing, a portfolio of thousands of songs, including rights to 259 songs by John Lennon and Paul McCartney that are considered some of the most valuable properties in music.

In 1985 Mr. Jackson paid $47.5 million for ATV, which included the Beatles songs — a move that estranged him from Mr. McCartney, who had advised him to invest in music rights — and 10 years later Mr. Jackson sold 50 percent of his interest to Sony for $90 million, creating a joint venture, Sony/ATV. Estimates of the value of the catalog exceed $1 billion.

Last year Neverland narrowly escaped foreclosure after Mr. Jackson defaulted on $24.5 million he owed on the property. A Los Angeles real estate investment company, Colony Capital LLC, bought the note, and put the title for the property into a joint venture with Mr. Jackson.

Trial Took Its Toll

In many ways, Mr. Jackson never recovered from the child molesting trial, a lurid affair that attracted media from around the world to watch as Mr. Jackson, wearing a different costume each day, appeared in a small courtroom in Santa Maria, Calif., to listen as a parade of witnesses spun a sometimes-incredible tale.

The case ultimately turned on the credibility of Mr. Jackson’s accuser, a 15-year-old cancer survivor who said the defendant had gotten him drunk and molested him several times. The boy’s younger brother testified that he had seen Mr. Jackson fondling his brother on two other occasions.

After 14 weeks of such testimony and seven days of deliberations, the jury returned not-guilty verdicts on all 14 counts against Mr. Jackson: four charges of child molesting, one charge of attempted child molesting, one conspiracy charge and eight possible counts of providing alcohol to minors. Conviction could have brought Mr. Jackson 20 years in prison. Instead, he walked away a free man to try to reclaim a career that at the time had already been in decline for years.

After his trial, Mr. Jackson largely left the United States for Bahrain, the island nation in the Persian Gulf, where he was the guest of Sheik Abdullah, a son of the ruler of the country, King Hamad bin Isa al-Khalifa. Mr. Jackson would never return to live at his ranch. Instead he remained in Bahrain, Dubai and Ireland for the next several years, managing his increasingly unstable finances. He remained an avid shopper, however, and was spotted at shopping malls in the black robes and veils traditionally worn by Bahraini women.

Despite the public relations blow of his trial, Mr. Jackson and his ever-changing retinue of managers, lawyers and advisers never stopped plotting his return.

As part of the agreement that saved Neverland, all of the property inside the house had to be removed quickly, and Mr. Jackson’s representatives hired a Los Angeles auction house, Julien’s Auctions, to prepare a sale of memorabilia from the ranch, according to court papers. More than 2,000 items were to be put up for sale in April 2009, and the value of the auction was estimated at up to $20 million.

But shortly before the sale, Mr. Jackson sued to prevent it, saying that he had never been given an opportunity to remove personal items. On the day the exhibition of the sale opened to the public — in a converted former department store in Beverly Hills — the auction house and Mr. Jackson announced that they had reached a settlement to return all of the property to him.

By early 2009, Mr. Jackson was living in a $100,000-a-month mansion in Bel Air, to be closer to “where all the action is” in the entertainment business, his manager at the time, Tohme Tohme, told The Los Angeles Times.

A spokesman for the Los Angeles Police Department said Mr. Jackson was not pronounced dead until about 2:30 p.m. The department assigned its robbery and homicide division to investigate the death, but the spokesman said that was because of Mr. Jackson’s celebrity. “Don’t read into anything,” the spokesman told reporters at the Bel Air house. He said the coroner had taken possession of the body and would conduct an investigation.

In another indication that the scrutiny never stopped, paparazzi were already waiting outside Mr. Jackson’s home when the ambulance arrived to take him to the hospital.