"The more I think about Michael, and talk about Michael, the more I think that 'King of Pop' is not good enough," Berry Gordy, the founder of Motown Records , said to the crowd.
"I think he is simply the greatest entertainer that ever lived.
"Though it ended way too soon, Michael's life was beautiful. Sure there was some sad times and maybe some questionable decisions on his part, but Michael Jackson accomplished everything he dreamed of."
More than 1.6m people had registered in an online draw to receive free tickets to attend the service, but only 8,750 were selected at random.
Thousands of other people also gathered outside the arena.
Police in the city of Los Angeles (LA) had earlier escorted the Jackson family and close friends to a private service at the Forest Lawn Cemetery in the Hollywood Hills ahead of the memorial service.
Jackson died on June 25 in Los Angeles after suffering cardiac arrest.
The 50-year-old entertainer was rushed to the city's UCLA Medical Centre after he was found unconscious at his home in the neighbourhood of Holmby Hills.
Jackson's cause of death would not be finalised until the results of toxicology tests are known in several weeks time, Los Angeles coroner's officials said after an initial autopsy.
No 'foul play'
Preliminary findings said there was no evidence of "foul play" or external physical injury.
Speculation has centered on Jackson's use of prescription drugs and reports that he was injected with the narcotic painkiller Demerol shortly before he went into cardiac arrest.
Jackson had been preparing for a series of comeback concerts starting July 13 at the O2 arena in London, England.
The performances were seen as the beginning of a public comeback by Jackson, who had spent recent years countering a series of damaging allegations about his private life.