Lady Gaga, Demi Lovato, More Light Up Z100 Jingle Ball

Kelly Clarkson delivers jaw-dropping set, as LMFAO, Gym Class Heroes, David Guetta and viral sensations Karmin help fill jam-packed MSG lineup.

NEW YORK — Without skipping a beat, Z100's annual Jingle Ball kicked off at exactly 7:30 p.m. Friday night to an arena full of screaming fans. The head-turning lineup featured Demi Lovato, Kelly Clarkson, LMFAO, Pitbull, Foster the People, Mindless Behavior, the Script, David Guetta, Gym Class Heroes and Lady Gaga, who brought some of the year's biggest hits to the Madison Square Garden stage.

ZI00's Elvis Duran and the Morning Crew personally came onstage to introduce the night's most-anticipated performance.

"Years ago, there was a young girl in the audience watching the Z100 Jingle Ball [and] she told her mom she wanted to be a singer," Duran said to the squealing concertgoers. "Three years ago, she opened up the show with her hit Just Dance. It gives me pleasure to say that a lifelong Z100 fan has now become one of the most important artists of our generation."

With the crowd chanting "Gaga," the New York City native opened with Edge of Glory. But in true Lady Gaga fashion, she didn't appear onstage right away. Instead, she emerged from a Christmas tree mid-song, decked out in a studded leather outfit. She then belted her way through "Bad Romance."

"When I was 11, I came to this very concert and told my mom I wanted to be a singer, and guys ... here I am," she said, launching into "Just Dance."

"Some people thought I was just going to be a one-hit wonder," Gaga continued as she moved on to mega-hits "Poker Face," "Telephone" and "Judas."

The pop diva then paid homage to the holidays with a naughty rendition of "White Christmas," gyrating her way across a holiday-themed set, complete with antlers, Christmas trees and lights.

Mother Monster danced through "Born This Way," before closing out the show with her latest, the Fernando Garibay-produced "Marry the Night", for which she channeled the video by changing into a a hospital gown and carting an IV stand onstage. As the song ended, fireworks went off and red confetti shot out all over the jubilant crowd.

Jingle Ball 2011's stellar lineup was more than just Gaga, of course.

During a short break in the show, fans took to their phones and began tweeting furiously in anticipation of another act: Demi Lovato. Finally, after what seemed like years in tween time, Big Time Rush introduced the 19-year-old, who took the stage looking classy in an all-black, gem-studded blazer.

Lovato, joined by her backup dancers, opened with "All Night Long," and followed up with her new single, Give Your Heart a Break, to the delight of screaming Lovatics. Demi turned serious, though, for her comeback ballad "Skyscraper," and the whole room sang along. She closed out her set by ripping off her blazer, all the better to shimmy to and sing the finale, "Unbroken."

"Thank you guys!" she told her adoring fans. "You all have been so amazing!"

Turns out Kelly Clarkson, who took it back with her massive uptempo hit "Since You Been Gone," is also a Lovatic. "I've been waiting for a long time to do a duet with Demi," Kelly said. "Wow, she can sing her butt off!" she added as Demi returned for the classic "Have Yourself a Merry Little Christmas." Needless to say, the two power singers ... killed it. (Lovato has long cited Clarkson as an inspiration.)

Clarkson then switched gears, dipping into her latest album, Stronger, for the sassy "Mr. Know It All," and chart-toppers like "My Life Would Suck Without You."

Gym Class Heroes opened with their staple "Cupid's Chokehold," but the hip-hop/rock band also rolled out numbers from their new album, The Papercut Chronicles. They said goodbye with frontman Travie McCoy rapping his solo hit "Billionaire," as the crowd eagerly sang Bruno Mars' hook.

Reality star Khloe Kardashian was one of the night's guest hosts, and she told MTV News she has "a passion for music." Dressed in LMFAO-approved leopard tights, Kardashian introduced the electro-pop duo, who were short one party-rocker. "SkyBlu couldn't be here with us tonight because he hurt himself wiggling," RedFoo explained.

Although he was without his partner in crime, RedFoo held it down, kicking off with "Sorry for Party Rocking" as he danced with two giant plastic zebras in the background. Foo went on to perform "Shots Shots" and "I'm in Miami" (a New York version, of course). He finished up with explosive -- there were fireworks -- versions of the LMFAO hits "Party Rock Anthem" and "Sexy and I Know It," as fans wiggled in their seats.

Youtube sensations Karmin also performed -- but not onstage. They surprised everyone by rocking right in the crowd.

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