South Korean authorities barred a protest by activists aiming to send propaganda leaflets across the heavily guarded border on Monday after North Korea threatened a “merciless” attack, underlining the intensified tension between the estranged neighbors.

Roadblocks prevented the activists from getting to the launch site near the border where they planned to launch balloons carrying DVDs, USB drives and leaflets with pro-democracy essays and news of political events, AFP reports.

The protest had been initially authorized by the government but was shut down by authorities – a move widely seen by analysts as Seoul’s efforts to prevent an all out conflict ahead of December elections.

“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 Sang-Hak, one of the organisers, told AFP.

North Korea had threatened to attack in the past but Pyongyang’s warning on Friday took on an unusual force, specifically naming where and when it would strike. “The surrounding area will become targets of direct firing,” the Korean People’s Army said in a statement on Friday, adding that local residents should evacuate.

South Korean troops are on high alert and deployed additional artillery and tanks to forward border positions, Yonhap news agency reported.

“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-Korean relations,” a Unification Ministry spokesman told AFP.