Mock soldiers armed with fake guns created a checkpoint on the Irish border as a warning to British Prime Minister, Theresa May, about the dangers of a hard Brexit.
Hundreds of demonstrators were at the event during which they also demolished a temporarily-constructed concrete block wall, built for the protest.
Anti-Brexit activist, Tom Murray, said: "It's the ordinary people of Ireland who won't accept the border, so whether they will physically attack a border or not I don't know. But they certainly won't accept one.
"But the reality is that this isn't about breaking down the border, this is about reminding people not to impose a border."
They claim a hard frontier could destroy the 20 year-old peace deal.
Both the British and Irish governments have said they hope to avoid a hard border after Brexit but some analysts suggest that, if customs checks are introduced, security forces may be needed to protect them.
The Police Service of Northern Ireland has been given extra resources for Brexit but says it hopes it can adopt a softer community-style approach.