South Korean police block a highway near the border, stopping activists from sending propaganda leaflets to the North.
Protesters had planned to launch anti-Pyongyang leaflets across the border using balloons.
But they were banned after North Korea threatened to attack if the launch went ahead.
Despite the warnings, the group, led by North Korean defector Park Sang-hak, urged the authorities to let them carry on.
SOUNDBITE: ANTI NORTH KOREAN PROTEST LEADER PARK SANG-HAK SAYING (Korean):
"Our aim is to send letters containing truth and facts to our 20 million North Korean brothers. Why does President Lee stand by Kim Jong-il and Kim Jong-un and stop us?"
A group of anti-war demonstrators joined the altercation, calling for leafleting to stop as it threatened the South's safety.
The government condemned the leaflet launch, and called for restraint.
Amid rising tensions on the border, South Korea says it will retaliate against any attack from the North.