PSNI and Garda officers were out in force at the border near Newry yesterday to keep an eye on a cross-border anti-lockdown motorcade.
An anonymous poster circulating on social media had called for motorists to take part in a 'Car Convoy' from Dublin Airport to Newry and back to the GPO in Dublin's O'Connell Street.
"Come and be part of history," the poster urged.
"Get the tank full and take a Sunday Spin like no other. North and South come together for freedom."
A social media video showed a tailback of cars at a roundabout outside Newry.
In another video clip, an unidentified male voice says: "We're been up the North and we're after having about 30 armed police surrounding us. They brought us into a car park and tried to get all our names.
"The Guards have the roads blocked off. They have a checkpoint there. They're doing everything to break us up."
The convoy was applauded on social media by activist Gemma O'Doherty, who leads a group in the Republic of Ireland opposed to Covid-19 restrictions.
"Massive turnout for North-South End Lockdown Convoy today," she posted on controversial social media site Parler.
But in a statement, the PSNI said fewer than two dozen cars were involved in the exercise.
"At around 12.30pm today, approximately 20 vehicles in convoy, most with multiple occupants, were stopped by police between the border and Newry.
"Those present were advised that their actions were likely in breach of both the Public Processions (NI) Act and the current Health Protection Regulations and accordingly, that they risked prosecution.
"After some consideration, those present elected to return towards Dublin.
"Where people do not follow the Health Protection Regulations, our approach will remain the same - we will engage with people first explaining the guidance and the law and encourage people to do the right thing.
"However, where we are left with no choice but to enforce, we will take this measure in order to protect the most vulnerable members of our communities."