Thousands of ethnic Hungarians have demonstrated in Slovakia, to protest against a new law that limits the use of minority languages there, BBC reported.
Only Slovak can now be used in public offices, and in institutions like schools and hospitals.
Slovakia says the move is in accord with European standards, but protestors argue it breaks international laws.
The Hungarian government says it has turned to international human rights organisations for help.
The Hungarian and Slovak prime ministers are due to meet next week, to try to defuse worsening relations.
More than half a million ethnic Hungarians live in Slovakia, who regard the new law as the latest in a series of crackdowns by the Slovak government against their culture.
Peter Pazmany, of the opposition ethnic Hungarian Coalition Party in Slovakia, said the law: "makes no sense... [it] only creates tension between people who have lived peacefully side by side".
Fines
Anyone found to be regularly misusing the Slovak language in public office now faces a fine of up to $7,000 (£4,300), the equivalent of nearly a year's average pay in Slovakia, reports say.
The Organisation for Economic Co-operation and Development (OECD) has said the new law respects the rights of minorities, but has noted the concerns and risks related to its enforcement.
Ties between Slovakia and Hungary have long been strained over Bratislava's treatment of its ethnic Hungarians, who make up about 10% of the population.
For its part, Slovakia has previously voiced its distaste over what it sees as efforts by Budapest to promote Hungarian culture within its own borders.
The protests over the new law follow a row last week, when Slovakia barred the Hungarian president from making a controversial visit.
President Laszlo Solyom had planned to visit a part of Slovakia with a large ethnic Hungarian population, to unveil a statue of the first Hungarian king, Saint Stephen.
Slovakian Prime Minister Robert Fico said the plans were a provocation to his nation.
The BBC's Nick Thorpe in southern Slovakia says relations between the two countries are at the lowest point for many years.
Hungary once ruled Slovakia, within the then Austro-Hungarian empire, until the end of the First World War and the eventual break-up of those territories.