On July 22, 2011, Anders Behring Breivik set off a bomb in a van near the government headquarters killing eight people, before gunning down 69 others, mostly teens, at a youth wing summer camp on the island of Utoya.
Survivors, bereaved relatives, Crown Prince Haakon and government officials attended the ceremony in Oslo on Sunday.
"July 22, 2011 was a brutal attack not only on individuals, but also on our democracy," said Merete Stamneshagen, head of a support group for victims and survivors.
The memorial, located in the capital's renovated government district, features a 12-metre-high mosaic depicting a small wading bird and blades of grass reflected in water.
The artwork was created by Norwegian artist Matias Faldbakken, who said he had seen the bird on Utoya.
The names of all 77 victims are engraved on the memorial.
Crown Prince Haakon also visited the new 22 July Centre, Norway's national memorial and education centre dedicated to the attacks.
Breivik was sentenced in 2012 to 21 years in prison, then Norway's maximum sentence, which can be extended indefinitely if he is still deemed a threat.
Now 47, the far-right extremist said he killed his victims because they supported multiculturalism.
