Norway on Saturday marked the sixth anniversary of twin bomb and shooting attacks that killed 77 people, pledging to keep the memory of the events alive and to combat intolerance.

The low-key commemorations opened at Oslo's government quarter, where Prime Minister Erna Solberg said: "For us, July 22 [2011] will always be a date connected to fear, despair and grief."

It was also "the day our values were put to the test," she added. "The day we were reminded what to defend and fight for: Trust, openness, diversity."

She stressed the importance of memorial sites and the need to "transfer knowledge about what took place."

Right-wing extremist Anders Behring Breivik was sentenced to 21 years in prison in August 2012 for the bomb and shooting attacks in Oslo and at a Labour Party youth camp near the capital. They were the worst acts of violence in Norway since World War II.

Mani Hussaini, leader of the Labour Party youth wing, underlined the need to learn about the ideology and hatred that fuelled Breivik.

"July 22 was not a tragedy, it was terrorism that targeted our democracy," he said.

Later in the afternoon, youth wing members and others gathered to remember their loved ones on the island of Utoya, where Breivik killed 69 people, mainly youngsters, in a shooting spree.

Wreaths were laid at both ceremonies and the victims' names were read out. Attendees included Labour Party leader Jonas Gahr Store and NATO Secretary General Jens Stoltenberg, premier at the time of the attacks.