About two years ago, without ever playing a live show, the Las Vegas-based band recorded a demo with lead-singer Brendon Urie, who had little vocal experience. On a whim, the band sent the demo to Fall Out Boy bassist Pete Wentz, who traveled to Sin City and signed Panic! to his Decaydance label.

After seeing Panic! At the Disco's circus-themed show at Jobing.com Arena in Glendale, AZ, Thursday (12/7), it's hard to believe that the band, which melds pop-rock, electronica and emo, never played a live gig before it slipped under Wentz's wing and released "A Fever You Can't Sweat Out."

At the show, set under a big top, Urie wasn't kidding when, after "The Only Difference Between Martyrdom and Suicide is Press Coverage," he announced, "Ladies and gentlemen and children of all ages, you're about to see an amazing spectacle of sight and sounds. Sit back and indulge in a show you won't soon forget."

The four-piece group was bolstered by two additional musicians who played cello, xylophone and keyboards atop circus props; a man and woman on stilts; and a handful of scantily clad dancers. Drummer Spencer Smith was placed high atop a carousel prop. Urie's face, like the rest of the band's, was made up as a clown. Contortionists accompanied Panic! At the Disco as they sang "Time to Dance." All of the band members wore vintage clothing.

The crowd roared when Urie, whose vocals were beyond impressive, stepped aside to a piano for the song "London Beckoned Songs About Money Written by Machines." "Camisado" found Urie playing guitar. "Lying is the Most Fun a Girl Can Have Without Taking Her Clothes Off" was a sexy romp followed by an instrumental segment during which women danced with ribbons and swaths of fabric. During "But It's Better If You Do," modestly dressed women jumped rope innocently before stripping when Urie demanded: "I want to see your hands in the air now." One dancer laid on top of the piano while another rubbed against Urie. As the song ended, he smiled innocently, then frowned as the women left.

"Thank you ladies for showing us your talents," he said with the emphasis on "talents." Urie then showed off his dancing talent with his six dancers during an instrumental break in "There's a Good Reason These Tables Are Numbered. Honey, You Just Haven't Thought of It Yet."

The audience handled the lion's share of the vocals on the hit "I Write Sins Not Tragedies." "I Write Sins Not Tragedies." A bride and groom on stilts added drama to the song, and, as the audience yelled "whore," men and women came out from under the bride's dress.

To fill the insatiable, 70-minute set, Panic! At the Disco played two covers: Queen's "Killer Queen" and The Beatles' "Eleanor Rigby." Surprisingly, the junior high/high school-aged crowd sang along to both of the songs. (It's worth noting that "Killer Queen" is featured in the videogame "Guitar Hero.")

The openers, Jack's Mannequin [ tickets ] and Cobra Starship [ tickets ], were just as strong, proving they could headline large venues in their own right. Cobra Starship's lead singer, Gabe Saporta, set the mood by yelling "Ready for some a-- shaking?" Pushing the album "While the City Sleeps, We Rule the Streets," Cobra Starship kicked off its set with "Send My Love to the Dancefloor I'll See You In Hell (Hey Mister D.J.)" The charismatic Saporta encouraged fans to clap their hands to "It's Amateur Night at the Appollo Creed!"

The audience sang along to "Bring It (Snakes on a Plane)," recorded for the film "Snakes on a Plane."

Jack's Mannequin received a warm welcome, and kept the audience standing during its entire set. Lead singer/pianist Andrew McMahon, who recently survived leukemia, comes from the Tori Amos school of piano playing, taking turns straddling the stool, standing up, sitting with one leg bent, and sitting straight, which helped make for an exciting show. Bathed in blue light, the crowd shouted over McMahon's voice for "Dark Blue," from Jack's Mannequin's album "Everything in Transit."

McMahon said he was returning to a club in Tempe, AZ, in February but with his stage performance and deft singing ability, he has a sound that easily fills an arena.