King Harald joined lawmakers at solemn ceremonies in parliament Tuesday to mark the occasion as he resumed his official duties following heart surgery. Parades, concerts and other celebrations are underway in cities around the country.
On June 7, 1905, Norwegian lawmakers declared independence after decades of Swedish rule. The move led to military buildups in both countries, but the Swedish government backed down, and war was avoided.
United Nations Secretary-General Kofi Annan has called the peaceful transition a model for nations to follow.
Norway had been under foreign rule since the mid-1300s, when it came under the Danish crown. Norway was transferred to Sweden in 1814 after the Napoleonic Wars, in which Denmark sided with the eventually-defeated France.
Some information for this report provided by AP, AFP and Reuters.