The conservative prime minister added he would not call a snap national election as a result of the political crisis and ruled out using mediation to resolve it.
Thousands rallied in Madrid and Barcelona on Saturday in a last-ditch call for Spanish and Catalan leaders to stave off a national crisis amid Catalonia's threat to secede.
Sporting white shirts, protesters packed Barcelona's Sant Jaume Square where the Catalan government has its presidential palace, shouting "We want to talk!" and holding signs saying "More Negotiation, Less Testosterone!" and "Talk or Resign!"
Asked if he was ready to trigger article 155, Mr Rajoy told El Pais newspaper: "I don't rule out anything that is within the law".
The streets burst with yellow and red, the colors of both the Spanish and non-separatist Catalan flags, and many carried the flag of the European Union.
Political instability in Catalonia has already caused major banks and companies to relocate their headquarters to other parts of Spain and has polarized Catalan society, where roughly half of the citizens want to stay in Spain. In particular, the Spanish federal government filed a complaint with the country's Constitutional Court over the Catalan government and parliament approving the law on the independence vote.
"It's reached a turning point and we need to get actively involved in the defence of Spain's values as a nation", added 52-year-old Joaquin Penas, an off-duty cavalry colonel with a Spanish flag draped round his shoulders.
Catalan leaders have said more than 90 percent of poll-goers voted for secession, but turnout was limited to 42 percent of eligible voters, as police occupied many polling places. Madrid responded to the vote with force, sending thousands of police to the region to shut down the vote.
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Raul del Hoyo, 56, a logistics technician, said he feels Spanish as well as Catalan but believes in the region's bid for independence.
The crisis has raised fears of unrest in the northeastern region, a tourist-friendly area of 7.5 million people that accounts for a fifth of Spain's economy.
On October 1, Catalonia held the independence referendum, which is not recognized by the Spanish authorities.
Yesterday tens of thousands of demonstrators, many dressed in white, hit the streets of Madrid and other cities across Spain to demand dialogue to end the dispute.
The stark warning came days before Catalan leader Carles Puigdemont is expected to address the region's parliament, on Tuesday, when he could unilaterally declare independence.
Rajoy said that includes the application of Article 155 of the Spanish Constitution, which would allow the central government to take control of the governance of a region "if the regional government does not comply with the obligations of the Constitution".
Catalonia's referendum law establishes an "exceptional legal regime" that "prevails hierarchically overall norms which it may conflict with", meaning it overrides other laws.
"If there is a unilateral declaration of independence, that will be that".
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