More than 200,000 parents have signed an INTO petition demanding smaller classes for their primary school children, the union said yesterday.
Its general secretary, John Carr, said the response from every county in Ireland is "a powerful message to Government that parents are no longer prepared to tolerate overcrowded primary school classrooms".
He said: "It is a protest against the way Government policy is crowding out their children, and a wake-up call to the Minister for Education and Science to reduce class sizes as Government policy promised."
He added: "The scale of the response sends a clear message to Government that parents are not prepared to put up with broken promises any longer.
"This massive petition from parents shows that class size is now firmly on the political agenda. It is a problem that Government must address."
Irish primary schools have the second largest classes in the European Union.
This month it was revealed that the number of students in classes of 30 and more has actually increased in recent years.
The Programme for Government had promised that over the next five years classes for children under nine would be reduced to below the international best-practice guideline of 20 to one.
However, Minister for Education Mary Hanafin has signalled that this commitment may not be met because of new spending on special education and language training for foreign nationals.