Sacked Catalan President Carles Puigdemont is calling for peaceful "democratic opposition" to the central government's takeover of the region.
On Friday, Spanish Prime Minister Mariano Rajoy removed Puigdemont, took over the administration of the autonomous region and set a new election after Catalonia's parliament declared itself an independent nation on Friday.
"It's very clear that the best form of defending the gains made up until now is democratic opposition to Article 155," Mr Puigdemont said in a brief statement, referring to the constitutional trigger for the takeover.
But he was vague on precisely what steps the secessionists would take as the national authorities took control.
European countries, the United States and Mexico have rejected the Catalan declaration and expressed support for Spain's unity.
Mr Puigdemont signed the statement as President of Catalonia, demonstrating he did not accept his ousting.
The secessionists say a referendum on October 1 gave them a mandate for independence. Less than half of eligible voters voted in the ballot Madrid declared illegal and tried to stop.
Opinion polls show that more than half of the 5.3 million people eligible to vote in the wealthy northeastern region, which is already autonomous, do not want to break from Spain.
In another development, Catalonia's police force told its officers to remain neutral, a step towards averting possible conflict following doubts over how the Mossos d'Esquadra would respond if ordered to evict Mr Puigdemont and his government.
Some Catalan officers stood between national police and those trying to vote during the referendum.
Government buildings, the headquarters of national political parties, ports, airports, courts, and the Bank of Spain were being guarded, the Interior Ministry said.
The Madrid government also sacked the Mossos' chief, Josep Lluis Trapero.
Mr Trapero became a hero to the secessionists after his force took a much softer stance than national police in enforcing the ban. Spain's High Court banned Mr Trapero from leaving the country and seized his passport as part of an investigation for alleged sedition.
There was no trouble overnight and the streets of Barcelona were quiet on Saturday.
The main secessionist group, the Catalan National Assembly, has urged civil servants to mount "peaceful resistance" while a pro-independence trade union, the CSC, called a strike.
About 1000 people took part in a pro-unity rally in Madrid on Saturday and others turned out in the northern city of Valladolid - an indication of the resentment the independence drive has caused in the rest of Spain.