South Korean police banned activists from sending propaganda leaflets across the heavily militarised border into North Korea on Monday, following last week's threat of attack by Pyongyang if they went ahead.
Police and troops blocked highway exits to the launch site close the demililitarised zone that separates the impoverished North from its affluent neighbour to prevent activists from launching balloons carrying materials denouncing Pyongyang.
The North had warned on Friday that it would fire on the launch site close to its border if the leaflets were sent across the border.
"(South Korean President) Lee Myung-bak woke up this morning and got scared of some immature kid (North Korean leader Kim Jong-un) and decided to stop us," the event's organiser Pak Sang-hak said as he confronted police on the highway.
Defectors and anti-North Korea activists have frequently launched balloons carrying propaganda and cash across the border.
Friday's warning of a "merciless military strike" was the most explicit in months and the first since Kim Jong-un, believed to be 28 years old, took power in December 2011.
South Korea had said it would retaliate against any North Korean attack, raising tensions on the border which separates two countries that remain technically at war because the 1950-53 Korean war ended with only an armistice.
North Korea shelled a South Korean island in November 2010, killing civilians and prompting counter-fire from the South.