Spain's King Felipe has said Catalonia "is and will remain an essential part of 21st century Spain".
In an unusually emotional speech at a prize-giving ceremony, he said the Spanish government would resolve the fight over Catalonia's bid for independence through "legitimate democratic institutions".
"Spain will deal with this unacceptable attempt at secession by using the Constitution," he said.
"We do not want to give up that which we have built together."
It was only the king's second intervention in the Catalonia crisis and came ahead of an emergency cabinet meeting.
Prime Minister Mariano Rajoy is meeting with his Cabinet this morning to set out specific powers it plans to seize from the northeastern region, as its leader Carles Puigdemont refuses to abandon his threat to declare a breakaway state.
Autonomy is a hugely sensitive issue in Catalonia, which saw its powers taken away under Spain's military dictatorship.
Home to 7.5 million people, the region fiercely defends its own language and culture.
Catalonia's government currently runs its own policing, education and healthcare.
Reports suggest Mr Rajoy is preparing to potentially take control of its police force, the Mossos d'Esquadra.
It could also force new elections in the Catalan parliament, where separatists have held a majority since 2015.
Speaking at an EU summit in Brussels on Friday, Mr Rajoy said a "critical point" had been reached and that his government had to act to stop the rule of law being "liquidated".