According to a the Associated Press, a picture of a young Michael appeared on stage while Madonna was performing "Holiday." During the number, a dancer dressed as the King of Pop came on stage in black pants, a sparkly jacket, white socks and a single white glove.
A host of fans quickly hit their Twitter accounts with further updates about the tribute.
According to several who claimed to be at the show, Madonna mixed Michael's "Billie Jean" and "Wanna Be Startin' Somethin'" in between her tune.
"Biggest cheer of the night at Madonna at o2 was when Jacko appeared and did a medley of hits," Absolute Radio UK DJ Ben Jones wrote on his Twitter account. "Hard to believe he was due to start in a week."
In fact, Michael was set to kick off his "This Is It" run of shows at the O2 arena in just nine days.
On Friday, a photo was posted on Madonna's Web site, showing the pop singer and her dancers rehearsing with a man dressed up like Michael.
Madonna and Michael famously attended the Academy Awards together in 1991.
The King of Pop's death last week at the age of 50, hit Madonna hard.
"I can't stop crying over the sad news," she previously said in a statement. "I have always admired Michael Jackson. The world has lost one of the greats, but his music will live on forever! My heart goes out to his three children and other members of his family. God bless."
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