He said Monday`s Catalan vote was a "clear violation" of the constitution.
The motion called on the regional parliament to aim for independence within 18 months.
It gives the assembly 30 days to start legislation on a Catalan constitution, treasury and social security system.
Catalan nationalist parties secured a majority of seats in September elections but fell short of winning half the vote. They had said before the vote that they considered it a de facto referendum on independence from Spain.
Spain`s state prosecutor had called on the Constitutional Court on Wednesday to suspend the Catalan resolution immediately, the prime minister said after an emergency cabinet meeting.
Opinion polls suggest a majority of Catalans favour a referendum on independence, but are evenly divided over whether to secede.
The Constitutional Court, which was due to hear the government`s appeal later on Wednesday, is expected to rule against the Catalan motion.
However, the pro-secession parties had fully expected the motion to be declared illegal and as part of the motion argued that the court lacked legitimacy.
Two big separatist parties make up the "Together for Yes" ("Junts pel Si") coalition but they needed the help of the far-left CUP (Popular Unity) party to secure an absolute majority in the Catalan parliament.
Artur Mas, the acting Catalan president who has spearheaded the drive for secession, has been trying to win re-election but has failed to secure the approval of the far-left party.
The CUP has called for another Together for Yes candidate, Raul Romeva, to take over the leadership role.
Several parties oppose secession in Catalonia, including the Catalan Socialists and Citizens (Ciudadanos), a centre-right party which was born in the wealthy north-eastern region but has attracted increasing popularity across Spain.
Its leader, Albert Rivera, said earlier this week: "To those Catalans who want independence: the solution is not to break up the country, it is to reform it."
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