South Korean troops and riot police on Monday prevented activists from sending anti-Pyongyang leaflets across the border after North Korea threatened a "merciless" military response.

Groups of North Korean defectors had planned to launch balloons carrying 200,000 propaganda leaflets at 11:30 am (0230 GMT) from Imjingak Park on the border near the town of Paju, some 60 kilometres (35 miles) north of Seoul.

But they were stopped in their tracks by a roadblock of police vehicles and security personnel who refused to let anyone approach the area.

"For security reasons, the activists were not allowed to launch balloons from the park," a police official said.

There were some minor scuffles as the activists sought to push through the roadblock, and a number of them refused to leave even after the scheduled launch time had passed.

"This event has been authorised by the government. This is ridiculous," complained Park Sang-Hak, one of the organisers, who blamed President Lee Myung-Bak for caving in to pressure from Pyongyang.

"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.

On Friday, the North Korean army had threatened a "merciless military strike" on the park if the balloon launch went ahead and had warned local residents to evacuate the area.

"The surrounding area will become targets of direct firing," it said.

Such leafleting events 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, who took over the reins of the isolated state after the death last December of his father, Kim Jong-Il.

Troops in the South had been placed on high alert and Yonhap news agency reported the deployment of additional artillery and tank units to forward border positions.

The news agency said artillery units in the North had been observed preparing their guns for firing.

Seoul's defence ministry spokesman said the army was closely monitoring the situation and was ready to respond to any provocation.

"If the North attacks our people ... we will stage a harsh and thorough retaliation that targets not only the origin of attack but also its supporting forces," the spokesman told reporters.

The latest face-off came at a time of heightened cross-border tensions in the run up to South Korea's presidential election in December.

Pyongyang had reacted angrily to the announcement two weeks ago of a new US-South Korean 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 (propaganda) acts, in consideration of inter-Korea relations," a ministry spokesman told reporters.

Civic groups in the South regularly drop leaflets over the border with messages criticising the Kim dynasty and urging the North Korean people to rise up against repression.

The leaflets also carry news about rebellions in other parts of the world, including events of the recent "Arab Spring".