The remaining parts of the vehicle used by mass murderer Anders Behring Breivik are displayed at the 22 July Centre in the government quarter in Oslo, Norway, July 21, 2015. Reuters/Fredrik Varfjell/NTB Scanpix

Norway's Crown Princess Mette-Marit attends a memorial service at Oslo Cathedral, as Norway marked the fourth anniversary of the 2011 attacks, in Oslo July 22, 2015. Photo: Reuters/Torstein Boe/NTB Scanpix

Leader of the Labour Party Youth Organisation (AUF) Mani Hussaini speaks during a ceremony to mark the fourth anniversary of the shooting on Utoeya Island, July 22, 2015. Photo: Reuters/Fredrik Varfjell/NTB Scanpix

Norway's prime minister said July 22 will always remain a dark day in the nation's history, as she paused to remember the 77 people who were killed by right-wing extremist Anders Behring Breivik four years ago.

At a wreath-laying ceremony outside the new July 22 Centre in Oslo, Erna Solberg said the victims will never be forgotten and that "we remember them with love".

Breivik, who is serving a 21-year prison sentence, confessed to the bomb attack at government headquarters that killed eight people and a shooting rampage at a youth camp on Utoya island where he murdered 69 others.

Several ceremonies are being held throughout today, also attended by Nato secretary-general Jens Stoltenberg who was Norway's prime minister at the time of the attacks in 2011.

Mr Stoltenberg attended the opening of the July 22 Centre, which shows how Breivik carried out the cold-blooded attacks.

Visibly moved after coming out of the building, he said the exhibition plays an important role in honouring the victims.

He also joined Crown Princess Mette-Marit and other politicians at a memorial service in Oslo Cathedral, and attended the unveiling of a monument for the victims on Utoya.

Breivik, a self-styled anti-Muslim militant, claimed he was acting in defence of Norway by targeting participants at the youth camp, which was organised by the left-wing Labour Party he accused of betraying the country with liberal immigration policies.

It was the worst massacre in Norway since the Second World War.

Breivik's 21-year sentence for mass murder and terrorism can be extended for as long as he is deemed dangerous to society, which legal experts say means he is likely to be locked up for life.

PA Media