It seems there will be a unilateral proclamation. I don’t think Spain will accept this. Can Spain avoid this politically?
Stating the referendum was illegal seems to have been ineffective. Is it within the reach of the government to do this?
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At the moment Catalonia is ruled by its own Catalan government that has autonomy over this region, autonomy that was granted time ago by the Spanish government. That means that now they rule in lots of aspect of it but always under the Spanish law.
Catalonia is an Autonomous Community within the Kingdom of Spain...
...The Statute of Autonomy gives the Generalitat of Catalonia the powers which enable it to carry out the functions of self-government. These can be exclusive, concurrent and shared with the Spanish State or executives. The Generalitat holds jurisdiction in various matters of culture, education, health, justice, environment, communications, transportation, commerce, public safety and local governments. Catalonia has its own police force, the Mossos d'Esquadra, although the Spanish government keep agents in the region for matters relating to border control, terrorism and immigration....
...Most of the justice system is administered by Spanish judicial institutions. The legal system is uniform throughout Spain, with the exception of so-called "civil law", which is administered separately within Catalonia*
In case of proclaming the independence, the Catalan government is breaking the law and then the Spanish govern and justice can act against that. They can remove this Catalan autonomy, so the Catalan government will lose their powers and the Spanish government will take its place as direct government.
Of course, the Catalan government could not accept that and still govern in catalonia, but then the Spanish government could enforce the law (similar as what happened in the referendum day).
That could lead to detentions, protest and even fights between the Catalan and Spansih police. From this point it will depend in the willingness of each side to achieve what they want.
One important factor is that the Catalan government does not have at the moment the international support and therefore the Spanish government will not have preasure and could enforce the law as strong as they need. If catalonia had this international support (for example, they did a valid referendum and the "Yes" won and EU accepts it) then the Spanish government will face more problems to not lose catalonia.
Anyways, all of this is never sure and can change form day to day
Legally speaking, the central government can take direct control of Cataluna and put the government of Cataluna under arrest.
Ultimately they can send the army. Depending on popular support, it could cause a Civil War.
They can also negotiate and play the appeasement card. The Central government could gave some privileges in exchange for staying inside Spain. So both sides could claim victory.