The Los Angeles County coroner's office said following an autopsy there were no signs of trauma in Jackson's death, but that additional tests will be needed to determine exactly how the singer died.

"The cause of death has been deferred," said Craig Harvey of the coroner's office. "The medical examiner has ordered additional testing such as toxicology and other studies."

Harvey said the tests would take four to six weeks to complete.

The outpouring of grief continues following Jackson's death with hundreds of fans joining in a spontaneous moonwalk, Jackson's signature dance move, in London on Friday.

The gathering, which took place outside the Liverpool Street entrance to the city's subway system, was arranged using social networking sites like Facebook and Twitter.

Performers at the Glastonbury rock and pop festival in southwestern Britain also lined up to include tributes to Jackson in their songs.

Australian teenage singer Gabriella Cilmi, who lives in London, became the first artist to pay tribute to Jackson at the annual festival when she slipped a few lines from his hit song "Billie Jean" into her song "Sweet About Me."

Meanwhile, former Beatle Paul McCartney led entertainers, musicians, statesmen and legions of fans around the world in paying tribute to Jackson.

McCartney, who performed a duet with Jackson in the 1980s, said he was "privileged to have hung out and worked with Michael."

"It's so sad and shocking," the ex-Beatle said. "He was a massively talented boy-man with a gentle soul. His music will be remembered forever and my memories of our time together will be happy ones."

Fans flock to Berlin wax museum

In Berlin, some fans headed to the Madame Tussauds wax museum, to see a life-size recreation of Jackson.

Tussauds let fans be photographed next to the figure depicting the former star in a glittering costume at the height of his fame, and provided a condolence book for visitors to sign.

Also in the German capital, the exclusive Adlon hotel, where Jackson notoriously hung one of his toddlers out of an upper-floor window, said it "greatly regretted" the death of their former guest.

Bee Gees star Robin Gibb called Jackson a "wonderful, sensitive human being," but bemoaned the criticism the singer had come under later in life.

"If even a small portion of the praise that is bestowed on Michael Jackson now in death was given to him last year, in life, he might well still be with us," he said.

"This tragedy should teach us a lesson to value and praise those gifts while we still have them in the world."

A sign of Jackson's undimmed star power came from the fact that 24-hour news channels ran wall-to-wall coverage of his death, while newspapers cleared their front pages for pictures of the late star. So great was the rush of Internet searches for news on the singer, Google said it initially thought it was under attack from a virus.

Cause of death unclear

Website tmz.com, quoting family members, reported that Jackson died after being given "a heavy does of morphine."

The injection was reportedly administered by his personal physician, who has gone missing since the singer's death, tmz.com reported.

The website, which was the first to report Jackson's death Thursday, said family members had been alarmed at his state and planned to admit him to a rehab facility for his morphine and prescription drug addictions.

The report said that in recent weeks Jackson had been too lethargic to rehearse properly for his sold-out comeback shows in London in July. The performances were an attempt by Jackson to revive his career and improve his financial situation.

Descent of a star

Born in 1958 in Gary, Indiana, Jackson became a child star as a member of the Jackson Five. As an adult, Jackson moved on to a successful - and often controversial - solo career. This career peaked with his 1982 record "Thriller," which is the best selling album of all time.

In later life Jackson struggled with a series of court cases and allegations about his private life, damaging his career and his financial situation, but he remained a popular character with an enormous fan base.

The online version of German news magazine Der Spiegel wrote that Jackson had two faces: One was "the monster, more alien than human, lifted, pale and in the end as emaciated as a skeleton."

On the other hand, there was "the genius that reinvented pop, sold more albums than anyone else … and was admired by the masses - in particular by his musical heirs, who never would have existed without his influence and who are now mourning his passing with an unequalled flood of condolence: Beyonce, Usher, Justin Timberlake, to name three of many."

"A comeback has seldom been as necessary as it was in Jackson's case," wrote German daily Sueddeutsche Zeitung.

Though it's now too late for a comeback, the troubled pop icon leaves behind a generation that has at some point tried to moonwalk and a legacy of young musicians - like Beyonce and Timberlake - who are carrying on in his footsteps.

mrm/dpa/Reuters/AP

Editor: Sonia Phalnikar