Catalan President Carles Puigdemont says Catalonia will not accept Madrid’s plan to curb the region’s autonomy.
In a statement, he described the imposition of direct rule as the worst attack on Catalonia’s institutions since the Franco dictatorship.
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”.
Earlier Mr Rajoy said he was triggering article 155 of the Spanish constitution, which allows for direct rule to be imposed in a crisis on any of the country’s semi-autonomous regions.
Speaking after an emergency cabinet meeting on Saturday, Mariano Rajoy stopped short of dissolving the region’s parliament but put forward plans for elections.
It comes after the Catalonia regional government held a referendum on 1 October to ask residents of the region if they wanted to break away from Spain.
Of the 43% of Catalans said to have taken part, 90% voted in favour of independence. But many anti-independence supporters boycotted the ballot, arguing it was not valid.
Mr Puigdemont and other regional leaders then signed a declaration of independence, but immediately suspended it in order to allow for talks.
He then defied two deadlines set by the national government to clarify Catalonia’s position.