Catalonia "is and will be an essential part" of Spain, King Felipe VI said on Thursday, adding that everyone must respect the Constitution and the principles of parliamentary democracy.
Felipe’s intervention, the second during the constitutional crisis, came as Madrid plans to impose direct rule on the region after Catalonia's leaders threatened to declare independence.
Spain's Constitutional Court ruled that Catalonia's recent referendum on secession was illegal and the Spanish parliament has rejected the regional government's attempts to break away.
The monarch, who was speaking at an awards ceremony in the northern city of Oviedo, received a standing ovation for his comments.
Prime Minister Mariano Rajoy said on Thursday Madrid would announce measures this weekend to take control of Catalonia after agreements were reached with the country’s main opposition parties.
"The goal is double: the return to legality, and the recovery of institutional normalcy," Rajoy said at the end of a two-day EU Summit in Brussels, adding that European leaders shared his concern that Catalan separatist authorities have acted against the rule of law.
French president Emmanuel Macron blamed extremists for "feeding" on separatism, and has offered Rajoy his "full, entire support.”
Some 43% of Catalans voted in the referendum, with 90% backing independence. However, the majority of anti-independence voters reportedly boycotted the vote.
King Felipe had previously accused Catalan President Carles Puigdemont of breaking "democratic principles" and showing "disrespect to the powers of the state."