Troops backed by armored vehicles have sealed off areas around official residences and government offices in the center of South Korea's capital after police battled in the streets with students demanding democratic reforms.
Witnesses said at least 20 policemen and 29 students were injured, some seriously, when 50,000 students attempted to march on City Hall May 14. The clash betweeen militant students and police was the bloodiest since campus protests over martial law started at the end of March.
About 7,000 students staged night-long torchlit demonstrations at eight universities in defiance of martial law regulations as the May 16 anniversary of the late President Park Chung Hee's 1961 coup approaches.
Demonstrations have so far touched 31 South Korean universities this week.
Militant students demand the government lift martial law by May 15 or face nationwide street demonstrations. Police are on alert throughout the country.
Students at Yonsei University where 3,000 of them stoned 800 riot police May 13, said government warnings of possible North Korean troop movements were attempts to shift attention from legitimate demands for more democratic freedom.