PESTERA, Romania — Romania’s Constitutional Court on Friday canceled the final round of a pivotal presidential election with only two days before the vote, saying it needed to ensure the “correctness of the electoral process.”

The surprise decision, in a NATO member state that shares a border with Ukraine, came days after Romanian leaders raised allegations that “cyberattacks” had tried to undermine the vote. The court’s ruling was also the latest in a series of political upheavals across Europe, where right-wing and nationalist movements have surged this year.

The front-runner in Romania’s now canceled election had been Calin Georgescu, an ultranationalist whose victory in a first-round vote late last month stunned Romania’s political establishment.

George Simion, a far-right leader who had endorsed Georgescu, denounced the court ruling, saying “a coup is underway,” but he urged supporters not to take to the streets in protest. “The system must fall democratically,” Simion said.

The court gave no explanation for its decision on Friday, and it was not clear when a new first round would take place.

— The New York Times