SEOUL, SOUTH KOREA — South Korean activists floated balloons carrying tens of thousands of anti-Pyongyang leaflets into North Korea on Monday, eluding police who had disrupted an earlier launch attempt due to threats from North Korea.

North Korea’s military warned last week that it would strike if the South Korean activists carried through with their plan to fly balloons carrying the propaganda leaflets across the border. South Korea pledged to retaliate if it was attacked.

South Korean police, citing security concerns, had sent hundreds of officers Monday to seal off roads and prevent the activists and other people from gathering at an announced launch site near the border. Residents in the area were also asked to evacuate to underground facilities, according to local official Kim Jin-a.

Later, some of the activists moved to another site near the border that was not guarded and carried out the launch of the balloons. South Korea’s Defense Ministry said it was closely monitoring North Korea’s military movements but there were no suspicious activities.