A group of North Korean defectors have launched balloons carrying 120,000 propaganda leaflets toward their former homeland on Monday.
Pyongyang had threatened military retaliation before the planned launch by Members of Freedom Fighters for North Korea.
Blocked by police from reaching the initial launch site, some of the defectors secretly headed to an alternative launch site out of view of South Korean authorities, a history museum on Ganghwa, about an hour's drive west of the capital, Seoul.
Group leader Park Sang-hak says the launch from the original site, Paju, had been authorized by the government and that blocking it at the last minute was ridiculous.
He then questioned why South Korean President Lee Myung-bak would "stand with the North Korean leadership" in stopping the group's activity.
A spokesperson for the Presidential Blue House denies involvement in trying to block the balloon launch, calling it 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.
Some citizens living near the initial launch site had voluntarily evacuated to shelters, fearing a North Korean artillery attack in retaliation to the balloon launch.
A small group of peace activists in the area had demanded the balloon launch be halted, saying it could be the catalyst for war between the two Koreas.
Spokesman Kim Hyung-suk of 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.
"North Korea's threat to retaliate militarily about a planned event by a civilian group is totally inappropriate," said Kim.
South Korean media quote officials saying there were signs the North was readying artillery in the hours before the publicly announced balloon launch.
Defense Ministry spokesman Kim Min-seok says such information about North Korean forces' movements are a military secret.
"The defense ministry acknowledges North Korea's military is probably making preparations for what they announced last week," said the defense official, adding 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 have gone on high alert, increasing combat air patrols as well as deploying artillery and tank brigades. Some military vehicles were seen heading north into Paju Monday morning.
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.