Activists in South Korea have been stopped from sending balloons carrying propaganda leaflets over the border with the country's northern neighbour.
Groups of defectors from North Korea had planned to launch 200,000 leaflets denouncing Pyongyang, but were blocked by South Korean troops and riot police on Monday.
North Korean officials had threatened a "merciless military strike" in response to the launch, warning residents of the launch site's neighbourhood to evacuate the area.
"The surrounding area will become targets of direct firing," said a Pyongyang statement.
Scuffles broke out between activists and police at roadblocks surrounding Imjingak Park on the border, near the town of Paju, some 60 kilometres north of Seoul.
"This event has been authorised by the government. This is ridiculous," complained event organiser Park Sang-Hak, who blamed President Lee Myung-Bak for caving in to pressure from the north.
"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.
"We are not here to provoke a conflict but to convey the truth to North Koreans. President Lee will be remembered as a cowardly leader who succumbed to North Korean threats," Park said.
Such leafleting events, carrying messages criticising the Kim dynasty and urging the people of North Korea to rise up in rebellion, are relatively common - and North Korea has threatened action in the past - but Friday's statement was unusually strong with its specific naming of the time and location, coupled with the evacuation warning.
It was also the first time such a precise threat had been made under North Korean leader Kim Jong-Un, believed to be 28 years old, who took over the reins of the isolated state after the death in December of his father, Kim Jong-Il.
Tensions have been building on the Korean peninsula during the past two weeks, with Pyongyang reacting angrily to a new US-South Korea agreement to nearly triple the range of the South's missile systems, bringing the whole of North Korea within range.
Seoul's Unification Ministry, which handles cross-border affairs, called Monday for restraint from both Pyongyang and the activists in the South.
"We urge the North to stop the threats ... and have constantly asked the civic groups to refrain from such acts, in consideration of inter-Korea relations," a ministry spokesman told reporters.
The leaflets also carry news about rebellions in other parts of the world, including events of the "Arab Spring".
North Korea shelled a South Korean island in November 2010, killing civilians and prompting counter-fire from the South.
The two nations remain technically at war, because the 1950-53 Korean war ended with only an armistice.