Jackson, 50 when he died last June, kept pace with backup dancers half his age during rehearsals for such hits as "Thriller," "Billie Jean," "Beat It" and "Human Nature." The film was shot as Jackson prepared for a marathon concert stand in London that never happened.
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"He looked better than he did when he was 30," said Jessica Childs, a 21-year-old aspiring dancer who caught the Los Angeles premiere. "His voice stood out."
Four of Jackson's brothers - Jermaine, Marlon, Tito and Jackie - attended, saying afterward that seeing their brother on film filled them with love and pride.
"It's amazing to see him up there doing his thing," Jackie Jackson said. "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. That's why I keep this with me at all times in my pocket. It's a little token of him," he said, pulling out a white-glove key-ring fashioned after one of his brother's best-known accessories.
"It was closure for me," said Marlon Jackson. "And it was a moment where I just felt his spirit inside of me. And that made me feel good."
Performances in the film included a medley of Jackson 5 hits the singer originally performed with his siblings.
Most of the material was intended for Jackson's private use, but it now serves as the last bow of a performer who ruled the pop charts in the 1980s and later retired to a reclusive life amid allegations of child molestation.
The mood at simultaneous premieres around the world Tuesday and Wednesday was tearful yet celebratory. At the Los Angeles premiere near the arena where much of the rehearsal footage was shot, "This Is It" director and longtime Jackson collaborator Kenny Ortega introduced the film to the audience, calling it the "last sacred documentation of our leader and our friend."
"It was touching. Well done. It was beautiful," said Casey Gosh, 24, who was invited to the premiere by a friend. "It told his story. You really felt like you knew him. It was his final performance."
"I loved seeing him in action again," said David Montalvo, who saw "This Is It" in New York City. "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."
The footage revealed just how elaborate and demanding Jackson's comeback run of 50 planned concerts last July would have been. One segment showed how Jackson would have made a grand stage entrance inside a mechanical spider. Another, intended as a 3-D film accompaniment on "Thriller," featured an expansive graveyard set.
"We thought it was excellent. The concert we never saw," said Marilyn Morrison, who also saw the film in New York. "Just seeing all the moves, his original moves, just seeing him doing them again. Just wonderful."
Early reaction from critics was equally positive. Matt Soergel of The Florida Times of Jacksonville called it an "exuberant, astonishingly entertaining concert film." "Looks like the world has missed one helluva concert," wrote Kirk Honeycutt of The Hollywood Reporter. And Nekesa Mumbi Moody of The Associated Press said, "The amazing performances Jackson delivers in this film are not a result of camera magic, but Jackson's own."
The film already was getting repeat business. Gina Meconi and a friend attended the Los Angeles premiere then went to one of the first public screenings right after.
"I had tickets for this anyway, so I said let's go see it again," Meconi said. "It was awesome. I loved it. I thought it was going to be sadder, but it wasn't sad at all."
Elizabeth Gonzales slept outside overnight to buy tickets to see the film at the Regal. Wearing a fedora, a spangled glove and a red leather jacket a la "Beat It," she said the movie was worth the wait.
"I thought it was great," the 19-year-old said. "People were clapping, screaming. It was crazy. It's like he's still alive. People still scream for him. His music is still alive."
Before the Los Angeles premiere, Ortega wiped away tears as he greeted dancers and celebrity guests, including Paula Abdul.
In an "Early Show" report, CBS News correspondent Ben Tracy spoke with Abdul. Michael Jackson, Abdul said, taught her how to be a "consummate professional."
American Idol runner-up Adam Lambert told Tracy Michael Jackson "fused a lot of different styles together." Lambert added that Michael also brought theater back into pop music.
Among others attending were Will Smith, Jennifer Lopez, Paris Hilton, Neil Patrick Harris, Katy Perry and Motown Records founder Berry Gordy Jr.
Said Abdul afterward: "It was beautiful. It was sad. It brought you closer to who he was as a person."
It was the biggest cinematic blowout ever for a music film as "This Is It" opened for paying customers immediately after the premieres, with evening and midnight screenings in North America to middle-of-the-night and morning showings in Europe, Asia and elsewhere.
Distributor Sony, which paid $60 million for the film rights, opened "This Is It" in 99 countries. It expands to 110 territories this weekend.
"I was tossing and turning with nervous feelings all night. Michael Jackson is a hero in my life, and I cannot wait to see the final shot of Michael," said Noh Kyeong-ae, a 34-year-old accountant, sobbing at a movie theater in Seoul, South Korea.
The simultaneous showings around the globe were anchored by a star-studded premiere at the Nokia Theatre, a concert venue across the street from Staples Center, where many of Jackson's rehearsals - and his high-profile public memorial - were held.
The plaza in front of the Nokia Theatre was transformed into an elegant red-carpet arrivals area, with a dozen crystal chandeliers, displays of Jackson's past costumes and "This Is It" spelled out in giant letters.
Jackson memorabilia was on sale inside, from T-shirts proclaiming "I Love MJ" to key rings reading "King of Pop."
The film captures Jackson dressed with customary flamboyance, his fashion flourishes including military epaulets, sequins and gold-spangled pants.
Jackson backup dancer Misha Gabriel said the film is "such an honest and raw look at the creative process that at times it makes me think that maybe he wouldn't want people to see so much of the creative process before it was finalized. But I think that's the beauty of the film."
"It's Michael becoming great, perfecting his perfection, if that makes sense," said fellow dancer Nick Bass.
Some critics, Tracy pointed out, contend the film is exploitive, coming too soon after Jackson's death. But after seeing the film, most couldn't help singing Jackson's praises.
Body builder Lou Ferrigno said, "I loved it, it was great."
Some of Jackson's family and friends saw "This Is It" in advance. Elizabeth Taylor, a longtime friend of the pop star, posted her thoughts Monday on Twitter.
"It is the single most brilliant piece of filmmaking I have ever seen," she wrote. "It cements forever Michael's genius in every aspect of creativity."
The 77-year-old actress added that she "wept from pure joy at his God-given gift" and urged her fans to see the film "again and again."
Marlon Jackson told CBS News his brother's performance in the film was "bittersweet."
"I loved it," he said, "but it reminds me I will never see my brother again."
Clocking in at one hour, 51 minutes, the film was culled from more than 100 hours of footage that captures Jackson as a showman, a mentor coaching backup talent and a goodwill ambassador.
Near the film's end, Jackson and the crew hold hands as he gives them a pep talk about the London shows.
"It's a great adventure," Jackson tells his colleagues. "We want to take them places they've never been before. We have to bring love back into the world."