Air raid sirens blared, traffic stopped, fighter planes roared overhead and schoolchildren were hustled into subway stations Wednesday as South Korea, still tense after an artillery exchange with North Korea three weeks ago, held its biggest civil defense drill in decades. 'Such a large-scale drill was the first of its kind since 1975,' said the mayor of Seoul, Oh Se-hoon, shortly after the exercise ended. 'During previous drills, people were not forced to head to shelters or underground facilities. They were just encouraged to do so.' Oh, who said the drill was conducted in a 'swift but orderly' manner, added that 'fighter jets disguised as the enemy flying across the city to simulate airstrikes made the drill more realistic.' The nationwide exercise, which began at 2 p.m. and lasted 20 minutes, was organized after the North Korean artillery strike Nov. 23 on South Korea's Yeonpyeong Island. Two marines and two civilians were killed, and the episode rattled the peninsula and the region. The United States and South Korea quickly staged joint military exercises in the Yellow Sea, and live-fire military drills have continued almost unabated in the South ever since. South Korea used to hold civil defense drills on the 15th of nearly every month, although in the late 1990s, as North-South relations began to improve, the drills became less frequent and almost casual. When the sirens went off Wednesday, hundreds of students from nearby schools streamed into the Ahyeon subway station, on the No.2 Green Line in Seoul. Middle school students shouted and giggled, and several boys kicked a soccer ball on their way to a trackside shelter. One 11-year-old boy stopped at a Dunkin' Donuts shop inside the station to buy glazed tofu. But older students from Ahyeon Polytechnic School were more restrained. Wearing cotton face masks they walked quickly, led by placard bearers, from their school building into the underground station. 'They were a little scared and a little nervous,' said a teacher, Hyun Jung-ya, after the drill ended. 'Since Yeonpyeong they know we have a problem in this country. We talk to them about it. They are taking it seriously.' Most office workers went to shelters and basements in their buildings. Workers at the Samsung headquarters in central Seoul, for example, were sent to the basement cafeteria and sub-basements in the company's gleaming skyscraper. When the first sirens sounded, Oh, the mayor, said, he went to the City Hall subway station to check on the equipment and procedures there. 'Everything was perfect,' he said. 'All serene.' 