More than 150 lorry drivers have staged a protest at the border to highlight the importance of free-flowing movement.
A convoy of trucks made its way from Donegal and crossed the border into Co Derry.
The protest was brought to a standstill at the main border route as drivers and supporters turned out to show the "chaos" custom checks would bring.
There are about 13,800 border crossings a day.
George Mills, of the Irish Road Haulage Association, said: "We don't have to fill out any documentation to cross this border, which we need to do every day because of where we live.
"After Brexit, we will be crossing an EU/non-EU frontier and that's going to create so many problems.
"We want to show the world how easy it is now. The people of the UK don't seem to regard the border as important.
"Only 2pc of their trade crosses this border, but it's nearly 100pc of our trade that has to cross, so it's a really big concern to us.
"We don't want to go back to any form of regulation that impacts free movement.
"Our nearest port is Derry and Belfast, and they must bring up all our oil, coal, gas, animal feed and timber.
"The nearest alternative is 200 miles away in Co Limerick that can handle that amount of cargo.
"If a shipload of coal comes into Derry and it's bound for a non-EU country, it could possibly be of a different grade than coal coming into an EU country.
"How will they divide that coal or animal feed? It will cause a lot more problems than they realise."
Truck driver Tom Doherty said it was important to show what a hard border would be like.
Mr Doherty (57), who travels to Dublin every day from the Inishowen area to deliver fruit and vegetables, said truck drivers would face chaos at custom checks.
"I did it up to 1990 and it wasn't nice," he said.
"We don't want to go back to those days of queuing at the border."