Credit: JCalderon/Splash News Online

When an underage Miley Cyrus hit up Hollywood hotspot Voyeur just before midnight Thursday, the 17-year-old singer proved she really can't be tamed.

PHOTOS: Miley's raciest outfits

"She was constantly smiling and seemed to be in great spirits," an onlooker tells, adding that Cyrus "waved her hands in the air and danced seductively while shaking her hips" to songs by David Guetta and DJ Khaled.

Still, Cyrus -- clad in tight denim shorts and a black crop top -- remained on her best behavior throughout the evening.

"It was clear she knew she was being watched by others," the source says, adding that Cyrus and her girlfriends were flanked by a security guard at all times. "No alcohol was in her presence the entire night."

By 1:30 Friday morning, the Last Song actress and her friends decided to call it a night.

"Miley looked care-free and let loose with the girls," the source tells Us. "She looked happy and seemed to be taking advantage of her time out in the club."