Thousands of ethnic Hungarians are demonstrating against a new law which has come into force in Slovakia.
The law imposes strict limits and threatens stiff fines for the use of minority languages.
Only Slovak can now be used in public offices and in places like schools and hospitals.
Slovakia says the move is in accord with European standards, but the protestors argue it breaks international laws.
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 - about 10% of the population.
The BBC reports anyone found to be regularly misusing the Slovak language in public office now faces a fine of up to $US7000 - the equivalent of nearly a year's average pay in Slovakia.
Hungary previously ruled Slovakia, until the end of World War I and the eventual break-up of the Austro-Hungarian empire.