"The law and dialogue, these are the ways out of the situation in Catalonia... no one is above the law," Rajoy said, adding: "I want us to stay together."
His comments came a day after Catalan leaders said they would defy a Constitutional Court ruling that provisionally blocked the planned independence referendum on November 9.
Catalan pro-independence parties declared they were "united" on the issue, which threatens to trigger Spain's biggest constitutional crisis in decades.
Fired up by last month's independence referendum in Scotland - although voters there rejected independence - hundreds of thousands of Catalans have protested in the streets in recent weeks, demanding their own vote.
Proud of their distinct language and culture, many of Catalonia's 7.5 million inhabitants have long complained they get a raw deal from the government in Madrid, which decides how their taxes are spent.
Catalans defy Madrid on independence vote
Catalan pro-independence parties have united in a legal gamble to go ahead with a breakaway poll despite a court injunction by Madrid to stop the vote.
Catalan leaders are forging ahead with a vote on independence, defying a national government court challenge in their bid to redraw the map of Spain.
Spain's Constitutional Court provisionally blocked the plan for a vote, but parties in the Catalan region opted to launch a legal gamble on Friday.
"We have agreed to maintain the election decree so that citizens can exercise their right to vote on November 9," regional government spokesman Francesc Homs told reporters in Barcelona.
Catalan pro-independence parties declared they were "united" on the issue, after holding meetings to forge a common front in the tense standoff, which threatens to trigger Spain's biggest constitutional crisis in decades.
The central government has vowed to keep Spain whole in opposition to the drive for independence in Catalonia.
But fired up by last month's independence referendum in Scotland - although the Scots rejected a breakaway - hundreds of thousands of Catalans have protested in the streets in recent weeks, demanding their own vote.
In another move of defiance on Friday, Catalonia's moderate conservative government formally decreed the creation of a commission to supervise the ballot.
The national government will ask the Constitutional Court to suspend that decree, just as it has suspended other Catalan legislation over the vote in the past week, Deputy Prime Minister Soraya Saenz de Santamaria said.
"No one in Spain can say on their own authority what is legal and what is not," the Deputy Prime Minister told a media conference. "That is a matter for the courts.
"This government has an obligation to obey the law and to make sure it is obeyed, because it has an obligation to make sure everyone respects democracy."
But Catalonian leader Artur Mas vowed to push on with the bid for independence.
"We will forge ahead and we will do it together," he said.
