Hundreds of protesters have warned Theresa May that a hard Irish border risks destroying Northern Ireland's hard-won peace.
A mock checkpoint manned by actors dressed as soldiers and customs officers was constructed close to the frontier in Co Down on Saturday 26 January.
Machine gun-toting soldiers' "towers" were camouflaged in green and black and concrete blocks were craned into place across the road as a backdrop for a string of angry speeches by anti-Brexit campaigners from across the island.
If the UK leaves Europe without a deal, the free flow of goods could be disrupted by the creation of a hard frontier on the island, the European Commission has said.
Demonstrator Tom Murray, from Co Donegal, said it is Prime Minister Mrs May's responsibility to sort out the issue.
He said, "Ireland will not be made to suffer the folly of the Tory party.
"We are the ones who will be suffering for the mistakes made in Westminster.
"We will not accept this border, we demand that London sort out the problem that they created."
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It is more than 20 years since the 1998 Good Friday Agreement which largely ended decades of violence.
Mr Murray added, "All the peace and prosperity that we have enjoyed will be destroyed by a hard border.
"Communities could be dragged back into the old days of living in the shadow of someone else's border.
"We are the people who will suffer the most."
Security towers manned by the British Army in the hilly and remote area near the city of Newry were decommissioned in 2003 as it ended conflict era operations in Northern Ireland in support of the police.
The Irish and British governments have said they want to avoid a hard border after Brexit, and multiple sources have said Britain's withdrawal from the EU should not prompt a return to violence.
Some security sources have argued that if customs checks are put in place, police will be required to protect them and that could leave officers at risk from dissident republicans.
The Police Service of Northern Ireland has received extra resources for Brexit but have officially envisaged light-touch, community-style policing.
Dublin's Foreign Affairs Minister Simon Coveney this week said it would be difficult to avoid installing new infrastructure following a no-deal Brexit.