SEOUL — Korean troops and riot police blocked a group of North Korean defectors from launching balloons carrying propaganda leaflets to their former homeland. Pyongyang had threatened military retaliation ahead of the planned launch.

Organizers of the controversial balloon launch vow they will find a way to continue flying leaflets to North Korea.

The activists had planned to send 20,000 propaganda leaflets into North Korea by balloon.

Park Sang-hak, the leader of Freedom Fighters for North Korea, says his group’s event had been authorized by the government and blocking it at the last minute is ridiculous.

Park questions why South Korean President Lee Myung-bak is standing with the North Korean leadership in stopping their activity.

A spokesperson for the Presidential Blue House denies any involvement in blocking the balloon launch, saying it is a matter for the defense ministry and police.

President Lee is nearing the end of his five-year term. He cannot run again for the office and the election to succeed him is less than two months away.

Dozens of members of the group, composed of defectors from North Korea, found the roads to the Imjingak pavilion in Paju, north of the capital, blocked by authorities. Some citizens living near the planned launch site also had voluntarily evacuated to shelters.

After a standoff of several hours, in a steady rain, the defectors turned around.

Meanwhile, a small group of peace activists in the area demanded the balloon launch be halted, saying it could be the catalyst for war between the two Koreas.

Spokesman Kim Hyung-suk at South Korea’s Unification Ministry, which deals with matters relevant to the North, says organizers of such launches should exercise restraint, taking into consideration the situation between the two Koreas.

Kim says North Korea’s threat to retaliate militarily about a planned event staged by a civilian group is totally inappropriate.

South Korean media quote officials saying there were signs the North was readying artillery in the hours before the planned balloon launch.

Defense Ministry spokesman Kim Min-seok says such information about North Korean forces’ movements are a military secret.

Kim says the defense ministry acknowledges North Korea’s military is probably making preparations for what they announced last week. He adds that South Korea’s forces will carry out a harsh and thorough retaliation on the origin of any North Korean attack and the forces supporting it.

Local media reports say South Korean forces went on high alert, increasing combat air patrols by F-15K and KF-16 jets as well as deploying artillery and tank brigades.

Some military vehicles were seen heading north into Paju.

There have been numerous balloon launches in the past, but the North Korean objection to this latest one made a specific threat to directly fire on the area surrounding the event.

The two Koreas have no diplomatic relations. An armistice signed in 1953 halted a devastating three-year civil war but no peace treaty has ever been signed.