Armed police rushed to the scene after they received several reports of a shooting incident at the club just before 3am.
“When we looked into it closer we found that the shots fired were blanks,” police spokesman Kjell Hjulstad told newspaper VG.
“But the whole situation caused people to panic. Many believed it was live fire and that they were being shot at on the dance floor.”
No arrests have yet been made. Police are hoping video footage from the venue will help them track down the perpetrator, a man believed to be in his late twenties.
Empty cases found at the scene suggested the man had fired six blanks, police said.
One witness told broadcaster TV2 that the man took out a gun and started shooting after he was knocked to the ground inside the club.
Guests at the club then threw themselves to the floor as the gunman ran out the door, TV2 reports.