The government said it began arming police Thursday with about 13,000 M-16 rifles to defend their posts against attacks by radical protesters, a move critics said can only incite more violence.

Riots broke out after the tough measures were announced. Authorities said about 3,400 students, hurling firebombs and rocks, battled riot police Thursday outside campuses in four cities.President Roh Tae-woo on Wednesday ordered security forces to crack down on radicals and dissidents, calling them a major threat to political stability.