Members of the Norwegian royal family, including King Harald V and his wife, Queen Sonja, attended the memorial. At times during the service the royal couple appeared to be overwhelmed by emotion.
Jens Stoltenberg, prime minister, said at the cathedral service in Oslo: "Every single one of those we have lost is a tragedy in itself. Together, it is a national tragedy."
Only a few hundred metres away from the cathedral was the site of a bomb blast which killed seven people.
Police have arrested 32-year-old Anders Behring Breivik over the bombing and shootings on the island of Utoya where at least 85 people were gunned down. (Later revised down to 76)
But officers were investigating whether a possible second gunman took part in the country's worst attacks since World War Two.
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Norway holds a minute's silence to remember attack victims
Norway held a minute's silence to remember the victims of the country's worst peacetime massacre.
Hundreds of mourners laid flowers outside Oslo cathedral as the minute silence became five minutes.
The normally quiet country of 4.8 million is still reeling from the shock of Friday's attacks when a self-confessed mass killer detonated a car bomb in the capital Oslo, killing seven people.
He later traveled to an island where a Labor youth camp party was holding its annual meeting and went on a shooting rampage, killing up to 86 youngsters. (Later revised down to 76 )