They are organised by a new group Border Communities Against Brexit
Mock customs checkpoints have been set up at six border crossings today to highlight the impact a ‘hard Brexit’ could have on border communities.
The demonstrations have been organised by a newly formed group - Border Communities Against Brexit - who are protesting the impact Brexit could have on peace, jobs and the free movement of workers in Ireland.
The organisation is opposed to Britain's vote to leave the European Union, despite Northern Ireland voting against such a move in June's referendum.
The protests took place place at six locations from Derry to north Louth: the Belcoo/Blacklion border, Carrickcarnon, Moybridge, Aghalane, Lifford Bridge and Bridgend.
Border Communities Against Brexit spokesman, Declan Fearon said the old hard borders had divided people's farms, communities and parishes.
"Young people nowadays have never known of that," he said.
Around 30,000 people cross the border for work every day according to a recent Westminster committee report on 'Northern Ireland and the EU Referendum'
In Carrickcarnon, locals dressed up as customs officers at a temporary road barrier where they called motorists into a mock customs booth and searched vehicles beside fake British customs signs that demanded they stop.
Signs telling motorists they were entering the European Union free travel zone were erected on the other side of the border.
One of the largest demonstrations was at Carrickarnon, between Newry and Dundalk, where a lengthy tailback of traffic was created by the crowds.
Sinn Féin MEP Matt Carthy called for support for the action against Brexit.
Mr Carthy said: "The idea of a new EU frontier, stretching from Dundalk to Derry, is not something which is anyone’s interests.
"The prospect of customs tariffs and the restriction of the free movement of goods, services and people across the border is now of deep and growing concern, especially to businesses, workers, students and farmers in the border region."
"In June, the people of the North, voted to remain within the European Union.
"It would be unthinkable now to have one part of this island operating within the EU and another outside it. But that is exactly what the British Government is proposing.
"Any strengthening of the border or undoing of progress made during the peace process, is something which nobody in Ireland wants, and would breach the Good Friday Agreement.
"We must all stand together uphold the democratic wishes of the people of the North. We must stand against any bolstering of the border or the reintroduction of customs checks or other barriers."