PAJU, Gyeonggi Province (Yonhap News) -- Hundreds of South Korean police officers stopped dozens of North Korean defectors and activists from sending anti-Pyongyang leaflets across the border via balloon on Saturday as North Korea warned of "unimaginable consequences."

About 500 police officers prevented a truck carrying balloons and about 200,000 leaflets from accessing the defectors' launch site of Imjingak near the Demilitarized Zone that separates the two Koreas.

Police have vowed to cordon off the area to stop the leafleting campaign, citing safety concerns of South Korean residents near the border amid North Korea's recent threats.

Park Sang-hak, one of the key organizers of the botched attempt, said he will continue the campaign to tell the truth to North Koreans and eventually encourage them to rise up against their leader, Kim Jong-un.

"The leaflets are meant to tell facts and the truth to North Koreans in a peaceful manner," Park said in a news conference in Imjingak, along with other activists, including American activist Suzanne Scholte, who is in South Korea as part of a week-long event to campaign for improving human rights conditions in the North.

Park's comment came as Uriminzokkiri, a North Korean propaganda Web site, warned that "the leaflet spreading, masterminded by the United States and its puppets, will lead to unimaginable consequences."   

The Web site said merciless punishments should be given to Park and other human rights activists for escalating inter-Korean tensions, citing an article by Pak Jung-suk, a former North Korean defector in Seoul who fled back to her communist homeland last year.

North Korea's Uriminzokkiri warned last month that the launch site will be immediately blown away even if a single leaflet is sent across the border.

In recent years, the North has threatened to launch a "merciless military strike" against South Korea over its propaganda leaflets, condemning them as psychological warfare. Still, no actual attack has occurred.

Park, who has led the campaign for years, said Pak had joined him in floating the propaganda leaflets about four times when she lived in Seoul.

Park said his mother in Seoul was close to Pak as they lived in nearby apartments and said Uriminzokkiri must have written the propaganda article citing Pak's name.