The Catalan president, Carles Puigdemont, says the region will not accept Madrid’s plan for direct rule.
He described it as the worst attack on Catalonia’s institutions since General Franco’s 1939-1975 dictatorship, under which regional autonomy was dissolved.
Prime Minister Mariano Rajoy’s plans include the removal of Catalonia’s leaders and curbs on its parliament.
An independence referendum went ahead on 1 October, despite being banned by Spain’s Constitutional Court.
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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 of the Catalan parliament to debate a response to Mr Rajoy’s plans.
Addressing European citizens in English, he added that the EU’s founding values were “at risk” in Catalonia.
Barcelona police said 450,000 people protested in the regional capital earlier on Saturday, with many chanting “freedom” and “independence”.
What is the Spanish government planning?
Mr Rajoy said he was triggering Article 155 of the constitution, which allows for direct rule to be imposed in a crisis on any of the country’s autonomous regions.
Speaking after an emergency cabinet meeting, Mr Rajoy stopped short of dissolving the region’s parliament but put forward plans for elections.
He insisted the measures would not mean Catalan self-government itself was being suspended. Instead, he said, the plan was to remove those people who had “taken self-government outside the law and the constitution”.
The measures, which are supported by opposition parties, must now be approved by the Senate in the next few days.
Reports say Spain’s interior ministry is preparing to take control of Catalonia’s Mossos d’Esquadra police force and remove its commander, Josep Lluís Trapero, who is already facing sedition charges.
The government is also considering taking control of Catalonia’s public broadcaster TV3, El País newspaper reports (in Spanish).
What did the Spanish media say?
“Return to the past” is how Catalonia’s Catalan-language, pro-independence El Punt Avui sums up Saturday’s developments on its front page. In an opinion piece, Xevi Xirgo argues that Madrid is mounting a “coup d’etat” to submit Catalonia to central rule in a way not seen since the 18th Century.
In an editorial in El Periódico, Spain’s fifth-biggest newspaper which publishes both in Spanish and Catalan, Enric Hernàndez argues that Madrid responded in kind to “the aggression of the independence camp”, which ignored Spanish law to hold a “sham referendum”.
Hernàndez calls on Mr Puigdemont to agree to Mr Rajoy’s call for early regional elections as the best way out of the crisis or else face a “social revolt with unforeseeable consequences”.
Meanwhile El País, Spain’s biggest paper, a left-of-centre daily, endorses the government in an editorial entitled “The democratic state responds” and declares that Mr Rajoy is acting to “restore constitutional legality in Catalonia”. “Democracy not only has the right to defend itself but a duty to do so,” it says.