OSLO, July 22 (Reuters) - Norway on Thursday marks 10 years since anti-immigrant extremist Anders Behring Breivik killed 77 people in the worst act of violence in the country since World War Two. Breivik detonated a car bomb outside the prime minister"s office in Oslo, killing eight, before driving to Utoeya island and shooting 69 people at a Labour Party youth camp on July 22, 2011. A ceremony takes place in Oslo on Thursday outside what was once the prime minister"s office - an empty shell since the attack due to disagreements over how to rebuild it - attended by Prime Minister Erna Solberg, survivors and relatives of the victims. It will be followed by a service at the Oslo Cathedral during which NATO Secretary-General Jens Stoltenberg will speak. He was prime minister and leader of the Labour Party at the time of the attacks. At the end of the service, church bells across the country will ring at 1200 CET (1000 GMT) for five minutes. Later, a ceremony on Utoeya island will honour the 69 victims of Breivik"s shooting spree. The day will conclude with an evening ceremony in Oslo during which King Harald will speak. A group of survivors have set up a Twitter account @aldriglemme (Never forget) to re-post tweets about the attack as they appeared 10 years ago. Breivik, 42, is serving a 21-year sentence, which can be prolonged indefinitely if he is deemed a continued threat to society. Debate over the attacks has shifted over time. Survivors, many of whom were teenagers at the time, are now determined to confront the far-right ideology which was a catalyst for the attack. read more This is a departure from Norway"s response at the time, which emphasised unity and consensus, with Stoltenberg calling Breivik"s actions attacks on Norway and democracy. On Tuesday, a memorial to 2001 teenage hate crime victim Benjamin Hermansen was defaced with the slogan "Breivik was right", an act widely condemned by politicians and the public and which is being investigated by police.