The event’s organizer, Jeon Oh-hyun, said, “What we want is an official apology from North Korea.”

The persuasion campaign coincided with military exercises in which thousands of South Korean troops practiced fending off an attack from the North near the rivals’ tense border. The drill, which the army said was routine and unrelated to the ship attack, involved building and defending pontoon bridges that scores of tanks used to cross a river as helicopters buzzed overhead.