South Korean authorities on Monday stopped activists from sending propaganda leaflets across the border following threats from the Democratic People's Republic of Korea (DPRK) to open fire.

Police and army blocked access to Imjingak, a pavilion at a tourist park just south of the heavily fortified border, where the activists, many of them DPRK defectors, had planned to launch giant balloons carrying anti-Pyongyang leaflets.

Police also ordered evacuation of some 820 residents along the demilitarized zone separating the two Koreas.

The military is put on high alert and has self-propelled howitzers and rocket launchers on standby, according to local reports.

Activists have regularly launched giant balloons loaded with propaganda leaflets which Pyongyang has denounced as "an undisguised psychological warfare."

After the activists announced their latest plan last week, the DPRK's Korean People's Army threatened in a statement to carry out a "merciless military strike" on the South Korean territory "the moment a minor movement for the scattering is captured" in the border area.

In response, South Korean defense minister Kim Kwan-jin said the country will strike back and "completely destroy the source of attacks".

Tensions remain on the border two years after four South Koreans were killed in a DPRK artillery attack on a front-line island near the disputed western maritime border.

South Korean President Lee Myung-bak paid a surprise visit last week to the island of Yeonpyeong and reaffirmed South Korea's commitment to deterring provocation from north of the border.

The two Koreas remain technically at war with each other after the 1950-53 conflict ended with a truce, not a formal peace treaty.