Large numbers of soldiers and police were at the scene, some guarding construction equipment, a Reuters photographer there said.
Slovenia is the smallest country on a major route for refugees and migrants heading north on their way to Austria and then Germany.
About 180,000 people, many fleeing war in Syria and Afghanistan, have entered the Alpine state since mid October, when Hungary fenced off its border with Croatia and pushed the migrant route towards it western neighbour.
Prime Minister Miro Cerar said on Tuesday the border would remain open, but the fence would help control the flow of people.