More than two hours later, she collected another trophy, as Country Artist of the Year. "I'm just havin' the best time doin' this," she said.
Justin Bieber was a triple winner, as Digital Artist of the Year, Top New Artist and Fan Favorite – great excuses to hug and kiss his seat mate in the audience of Las Vegas's MGM Grand: Selena Gomez.
On stage, sporting a gold lamé tuxedo jacket, the "Baby" singer, 17, thanked his fans, as well as his mother and "my whole family, [talent manager] Scooter Braun, Usher – basically everyone who helped me get out of my hometown and live my dream."
As PEOPLE reported from the rehearsal, a ravishingly redheaded Rihanna opened the show with her song "S&M" before being joined a black-masked Britney Spears.
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One of his ensuing comedy sequences was followed by his movie costars Bradley Cooper, Ed Helms and Zach Galifianakis espousing their love of "urban music" – and introducing Keith Urban, who sang his country hit "Long Hot Summer" while wife Nicole Kidman smiled at him from the audience.
All-Star Beyoncé TributeSpears returned to the stage hours later, to duet with rapper Nicki Minaj. Rihanna returned when Justin Bieber handed her award for Female Artist of the Year. "It was so cool" to have him present it to her, she said.
Taio Cruz's "Dynamite" was named Top Hot 100 Song. Lady Antebellum took Country Song of the Year, for the heavily downloaded "Need You Now." The Black Eyed Peas were the Top Duo/Group. "I want to thank our families for dealing with us," said Fergie.
At the top of the second hour, no less than First Lady Michelle Obama, Bono, Barbra Streisand, Stevie Wonder and Lady Gaga paid tribute (on film) to Beyonce Knowles. Mrs. Obama applauded "the role model she provides so many women," while mother Tina Knowles spoke of how Whitney Houston had served as the young Beyoncé's own role model. "Beyoncé has made history and she's not even 30 yet," said Wonder.
Performing a dynamic rendition of "Run the World (Girls)" that took her right into the audience, Beyoncé was then presented the Billboard Millennium Award, for her music and for her foundation's humanitarian work. While expressing her thanks, Beyoncé also shouted out her love to her husband, Jay-Z.
Other notable winners during the performance-heavy, three-hour broadcast on ABC included Rihanna again, this time as Radio Artist of the Year; U2, as Top Touring Artist; and the Icon Winner, Neil Diamond. "I don't know what it means to be an icon, but I always wanted to be one," he said. "I'll Google it after the show."