Catalan leader Carles Puigdemont says Catalonia will not accept Madrid's plan to impose direct rule on the region.
He described it as the worst attack on Catalonia's institutions since General Franco's 1939-1975 dictatorship, under which regional autonomy was dissolved.
Spanish PM Mariano Rajoy's plans include the removal of Catalonia's leaders and curbs on its parliament.
It follows the independence referendum that went ahead despite being banned by Spain's Constitutional Court.
Mr Puigdemont said the Spanish government was acting against the democratic will of Catalans after refusing all offers of dialogue.
He said he would call for a session in the Catalan parliament to debate a response to Mr Rajoy's plans.
Addressing European citizens in English, he added that the European Union's founding values were "at risk in Catalonia".