The incident took place after a bomb attack on Capital Oslo that killed seven people, including officers of Prime Minister Stoltenberg.
Reports say the killer set off explosion in downtown Oslo and before heading before heading to the summer camp.
According to details, a gunman wearing a police uniform asked young people to gather round and then started shooting.
No group has claimed responsibility for the attack but the suspect is reported by local media to have had links with right-wing extremists.
Reports said the country has had problems with neo-Nazi groups in the past but the assumption was that such groups had been largely eliminated and did not pose a significant threat.
Oslo police have been questioning a 32-year-old Norwegian man in connection with Friday’s attacks.
The man was arrested on tiny Utoeya island outside Oslo, where police say he opened fire on teenagers.