Today is the big day. This evening at 6 p.m., Carles Puigdemont will appear before the regional parliament to discuss "the general political situation" in Catalonia. Most of the country expects that to translate as some kind of declaration of independence from Spain.

The precise wording of that statement, and to what extent it will be interpreted as a genuine declaration of independence by courts, prosecutors, the central government and Catalan separatists themselves, is unknown this Tuesday morning.

The size and reach of the Spanish state's response to any declaration of independence is also unknown: the options range from criminal charges of sedition or rebellion, through the suspension of home rule in Catalonia for an unspecified period of time and even to articles of the Spanish Constitution that allow for the declaration of a state of alarm or exception.

Given the policing shortcomings of the past 10 days, any move to arrest Mr. Puigdemont and members of his regional government would be fraught with potential difficulties.

The Catalan Police, the Mossos, have closed parliament park, the Parc de la Ciutadella, in central Barcelona, erected metal barriers around the parliament building and parked several dozen police vans outside.

The Catalan National Assembly (ANC), Omnium Cultural and local separatist groups are busy organising untold thousands of supporters to travel to the park and the area outside the High Court, in the adjacent road.

With a proven ability to mobilise several hundred thousand people, Spaniards will find out this evening if those policing efforts are enough hold back the protestors or if, in the end, they overflow into the park and surround parliament. Messaging shifted slightly overnight from "defend parliament" to "peacefully support" Catalan institutions.

Police—regional or national—were unable (national) or unwilling (regional) to stop most people who wanted to from voting on October 1, and did not stop separatist protestors from shutting down Catalan roads during a day of "total stoppage" last Tuesday.

On September 20, snap protests called by the ANC and Omnium led to the investigation of sedition because court officials and judicial police were blocked inside a building for several hours and Civil Guard SUVs vandalised.

The Spanish Home Office refused to comment on any National Police and Civil Guard provisions for the day, citing operational security requirements.