The deployment of more than 300 police came after the activists vowed to push ahead with their political leaflet launch at a park in the border town of Paju, some 40 kilometres north of Seoul.
The activists plan to release balloons carrying around 40,000 leaflets criticising the North's government across the heavily-militarised frontier.
Near the park, some 20 local residents set up a road block with two tractors, carrying a placard reading "Stop anti-North leaflet launch jeopardising our lives!".
Despite Seoul's stance that the activists have a democratic right to launch the leaflets, police have said they might intervene to prevent a clash between activists and residents.
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Pyongyang, which refers to the activists as "human scum", has long condemned the launches and in recent weeks has stepped up its demands for Seoul to ban the practice entirely.
Two weeks ago, North Korea border guards attempted to shoot down some balloons, triggering a brief exchange of heavy machine gun fire between the two sides.
The North has warned that failure to halt future launches could scupper the planned resumption of high-level talks between the two Koreas.
Local residents in Paju insisted that the threats of military retaliation by North Korea are credible and that the activists are putting their lives and businesses at risk.
The South says there is no legal basis for a blanket ban, but it has urged the activists to exercise common sense and restraint.