Jackson's brothers attended the screening in Los Angeles and simultaneous premieres were held in 18 other cities.
At that premiere, fan Ed Rahmen said the movie proved the concerts at London's O2 Arena would have been "the best thing he had ever done".
"It was like a film on stage. It was better than any of his early stuff. It was Jackson brought to this time and age."
Neda Allin added: "You can see he was a bit ill, or he didn't look right, but the movie was brilliant."
David Montalvo, who saw the film in New York, said: "I loved seeing him in action again. It's like you were able to see Michael again for the last time, so it was a good chance to say goodbye to him."
Marilyn Morrison, also in New York, said the footage was "excellent".
She added: "Just seeing all the moves, his original moves, just seeing him doing them again. Just wonderful."
The Los Angeles premiere took place across the road from the Staples Center, the site of Jackson's rehearsals and, later, his public memorial service.
Jermaine Jackson arrived with his brothers
The Jackson brothers were joined at the screening by stars including Will Smith, Jennifer Lopez, Paris Hilton, Katy Perry, Jennifer Love Hewitt and Motown Records founder Berry Gordy Jr.
Jermaine Jackson said he thought the film was "very positive, very entertaining" and it had made him feel "really, really proud".
Jackie Jackson, meanwhile, said: "It's amazing to see him up there doing his thing.
"To see him up there doing his performance like that has brought a lot of tears to my eyes, sitting there watching him. Because I love him so much."
And Marlon Jackson said seeing the film had given him "closure".
He added: "It was a moment where I just felt his spirit inside of me. And that made me feel good."
We have this piece of film to remind us forever and ever that once there was such a man. God kissed him
Elizabeth Taylor on Twitter
Speaking on website Twitter, Jackson's close friend Elizabeth Taylor called it "the single most brilliant piece of filmmaking I have ever seen".
"I wept from pure joy at his God-given gift. There will never, ever be the likes of him again.
"We have this piece of film to remind us forever and ever that once there was such a man. God kissed him."
In London, stars including Peter Andre, Spice Girl Mel B and boy band JLS attended the screening at Leicester Square.
Rapper Chipmunk said the film was "absolutely amazing".
He added: "It's given me the package of what his tour could have been if he was still alive."
THIS IS IT PREMIERES
New York City
Some fans protested outside the screenings, saying the film covers up Jackson's declining health.
The dedicated fans, who have set up a campaign called This Is Not It, are accusing concert promoter AEG Live of putting too much pressure on the star during the build up to his 50-date run at London's O2 Arena.
A spokesman for AEG declined to comment.
Jackson, who died on June 25 aged 50, had spent the previous four months rehearsing in Los Angeles.
More than 800,000 tickets had been sold for the concerts, with organisers promising one of the "most expensive and technically advanced" live shows ever.
He was just two weeks away from the opening night at the time of his death, which authorities in Los Angeles ruled a homicide.
This Is It has now been released to cinemas for a limited two-week run. The first public screening in London began at 0400GMT, with fans queuing through the night to see it.
By the weekend, it will be shown in 110 countries, with distributor Sony putting 15,000 prints into circulation.
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