South Koreans have stopped their cars and huddled in underground shelters during the country's biggest-ever civilian evacuation drill.
Wednesday's 20-minute exercise was a response to soaring tensions after North Korea attacked a front-line South Korean island with artillery last month. Two civilians and two military personnel were killed.
The National Emergency Management Agency expected up to 12-million people to join the training.
During the drill, South Koreans were asked to evacuate to underground shelters after stopping work or parking their cars along streets.
A dozen South Korean jet fighters simulated air strikes, and a bullet train ran at a dramatically lower speed.