Norway is paying homage to the 77 people killed in a bombing and shooting rampage five years ago, with church services and events to mark one of the darkest days in the Scandinavian country's history.

Prime Minister Erna Solberg described July 22 as "a very sad" day that shook the small nation. "It is always very onerous," she told Norwegian news agency NTB.

Solberg will lead government tributes at wreath laying ceremonies and at a service in Oslo Cathedral.

Also in attendance will be members of Norway's royal family, victims' families and representatives from a youth organization that organized the summer camp on the island of Utoya where Anders Behring Breivik gunned down 69 people after exploding a car bomb at the government quarter in Oslo that killed another eight.