Objects used by mass killer Anders Behring Breivik, including the van that concealed a bomb, have gone on show in Norway amid fears the exhibition could attract sympathisers.
The display in Oslo opened four years to the day after Breivik killed 77 people in a bloody rampage.
Officials says it aims to help Norway come to terms with the terror attacks.
But experts and some relatives of the victims have warned it could become a magnet for right-wing extremists.
Concern has focused on the inclusion of some personal items belonging to the killer, including a fake police identity card.
Last week, Tor Oestboe, whose wife was among those killed, said he feared the exhibition would provide a “hall of fame” for the mass murderer.
Breivik killed eight people in a bombing in central Oslo and went on to gun down 69 more at a youth camp at nearby Utoeya island in 2011.
He harboured extremist right-wing views and claimed he had reacted against what he saw as a Marxist-Islamic takeover of Europe.
Nato Secretary General Jens Stoltenberg, who was prime minister of Norway at the time of the massacre, attended commemoration ceremonies on Wednesday.
He said he believed the exhibition, housed in the government complex targeted by Breivik, was necessary.
“July 22 is a dramatic event in the history of Norway. It cannot be hushed up, but must be treated with the dignity and respect that such a brutal incident must be dealt with,” local media quoted him as saying.
Oeystein Soerensen, a political historian, told Norway’s Dagbladet newspaper last week that the exhibition could become “a kind of twisted Mecca” for anti-Islam and right-wing extremist “tourists”.
Mr Stoltenberg is due to attend the Labour Party youth camp on Utoeya island in August, which will take place for the first time since 2011.
A new memorial is also being unveiled on Wednesday to those killed on the island.
In 2012 a court in Oslo found Breivik guilty of a premeditated act of terrorism. and sentenced him to 21 years in jail.
He “acknowledged” the acts committed, but said he did not accept criminal responsibility.
His jail term can be extended if he is deemed to remain a danger to society.
He has since launched legal action accusing the Norwegian state of violating his human rights by keeping him in strict isolation in prison.
Earlier this month he won a place to study political science at Oslo’s university, although he will do so from his cell and have no contact with staff or students.