The activists, led by a North Korean defector, were prevented by police from launching anti-North Korean leaflet balloons towards the North.
A South Korean group led by a North Korean defector has clashed with police in the city of Paju as they tried to fly anti-North Korean balloons with leaflets in them over the border.
It threatened an agreement by the Koreas not to conduct hostile activities before an historic meeting between President Trump and North Korean leader Kim Jong Un.
Pyongyang has in the past blamed Seoul for failing to stop previous leaflet launches, calling them "an act of war". The two Koreas even traded fire in 2014 after the North's military shot machine guns at balloons launched by defector activists.
"Our society has forgotten the evil essence of the Kim dynasty and been carried away by a fake peace atmosphere, the defector and anti-North Korean activist Park Sang-Hak said at the event.
A group of peace activists was also present to counter the defector's group.
Seoul has been urging defectors and other anti-North Korean groups to stop hostile activities but they've refused saying that it's prefectly within their rights to do so.
On Saturday President Trump said a time and a place for his meeting with Kim Jong Un had been agreed on and would be revealed soon.