The protesters, who expressed strong support for the nation’s armed forces, held demonstrations all over the country on Sunday.
A government statement said North Korea’s nuclear weapons are “the nation’s life that will not be traded even for billions of dollars.”
The statement was published after North Korea’s leader Kim Jong-un took part in a meeting of the central committee of the ruling Workers' Party of Korea.
On Saturday, Pyongyang announced declared that it is in a “state of war” with South Korea, warning that any provocation by Seoul and Washington will trigger an all-out nuclear war.
Pyongyang said henceforth “the North-South relations will be entering a state of war and all issues raised between the North and the South will be handled accordingly."
"The long-standing situation of the Korean Peninsula being neither at peace nor at war is finally over," said a statement issued jointly by the government, political parties and other organizations.
Pyongyang also warned that if Washington and Seoul launched a preemptive attack, the conflict "will not be limited to a local war, but develop into an all-out war, a nuclear war."
On Tuesday, North Korea said that its military should be prepared to attack "all US military bases in the Asia-Pacific region, including the US mainland, Hawaii, and Guam" and South Korea.
The announcement came days after South Korea and the US signed a new military pact in response to what they called even low-level provocations by Pyongyang.
On March 11, Seoul and Washington launched a week-long annual joint military maneuver near the Korean Peninsula despite warnings from Pyongyang. The maneuver involved 10,000 South Korean soldiers and about 3,000 US troops.