Protests have taken place against the US-South Korean Key Resolve military drill, an annual joint military exercise by the two countries.
According to the Joint Chiefs of Staff, about 10,000 South Korean troops and 3,500 US forces are joining the drill, which is being held from 11-21 March.
A pro-North Korean activist group calling themselves Solidarity for Peace and Reunification of Korea protested outside the US embassy.
The group insisted that the decision to go on the annual drills under the current circumstances of the Korean peninsula will only intensify the danger.
"This kind of offensive practice raises tensions on the Korean peninsula," said the director of Solidarity for Peace and Reunification of Korea, You Young-jae.
After the start of the drills, South Korean officials said their northern counterparts did not answer two calls on a hotline between the sides, apparently following through on an earlier vow to cut the communication channel because of the drills.
According to a South Korean expert on North Korean issues, North Korea's provocative announcement on the Key Resolve drills seems less likely to cause an actual military attack.
Pyongyang has launched a bombast-filled propaganda campaign against the drills and last week's UN vote to impose new sanctions over the North's 12 February nuclear test.
North Korea regularly claims South Korea-US drills are a preparation for invasion, but Pyongyang has signalled more worry about the drills that began today.
The drills follow UN sanctions that the North says are the result of US hostility aimed at toppling its political system.
North Korea has also warned South Korea of a nuclear war on the divided peninsula and said it was cancelling non-aggression pacts.