Hungary said blocking Laszlo Solyom's visit went against EU rules
Budapest has summoned Slovakia's ambassador to protest at Bratislava's refusal to let Hungarian President Laszlo Solyom into the country.
Mr Solyom had been due to visit a region populated by ethnic Hungarians to unveil a statue of the first Hungarian king last Friday.
He was stopped at the border in a move that Hungary described as going against European Union rules of free movement.
Slovakia's government had already said the visit was inappropriately timed.
Hungarian Foreign Minister Peter Balazs described Slovakia's refusal to allow Mr Solyom into the country as "unprecedented and unacceptable", the Slovakian SITA news agency reported.
After his meeting on Monday with the envoy, Peter Weiss, Hungary called on the government in Bratislava to explain the incident, the report added.
'Violation of sovereignty'
Mr Solyom had been due to attend the unveiling of a statue of the medieval Hungarian king, Saint Stephen, in the southern town of Komarno, on the River Danube, which separates the two countries.
But Slovakia's Foreign Minister Miroslav Lajcak told the BBC no Slovak officials had been invited to Friday's event, which coincided with the anniversary of the 1968 invasion of Slovakia by five armies, including Hungary's.
Slovakian Prime Minister Robert Fico said any visit would be a "violation of international law and Slovakia's sovereignty".
The row comes at a time of heightened bilateral tension stemming from linguistic and historical issues, correspondents say.
Relations between the two countries have long been strained over Slovakia's treatment of its ethnic Hungarians, who make up about 10% of the population.