The public prosector's office in Catalonia said in a statement e-mailed to journalists on Tuesday that "the illegality of any acts directed towards holding the referendum is beyond all doubt".
Officers must now "urgently" prepare reports and statements on any activities they suspect of being related to the referendum, "urgently" inform the public prosecutor's office of the activities, "immediately" collect evidence of the crimes, and seize materials or instruments being used to prepare referendum materials or organise the vote.
The prosecutor's office said that any public servant or citizen aiding in the preparation of that illegal vote will now be liable to criminal investigation for contempt of court, abuse of authority and misuse of public funds.
At a meeting on Tuesday morning, the three police chiefs in Catalonia—Civil Guard, National Police and the Mossos, the Catalan regional police—were informed of their instructions in person.
Josep Lluis Trapero, the head of the Mossos, was present at the meeting.
They were ordered by the prosecutor to investigate and prevent any acts related to the preparation or organisation of the Catalan referendum, after being reminded of their constitutional function as judicial police: they must follow judges' orders.
"The Mossos d'Esquadra", said the prosecutor's office in its orders: "in the same way as the Civil Guard and the National Police, as judicial police, are obliged to follow instructions they receive from the Public Prosecutor related to the investigation of crimes and the prosecution of delinquents".
The three police chiefs have been given 24 hours to make sure all of their officers are aware of the policing instructions to stop the referendum from happening.
Similar meetings will take place tomorrow between prosecutors and police chiefs in the four provinces in Catalonia: Barcelona, Tarragona, Lérida and Gerona.
Asked about the prosecutor's orders to police chiefs, Catalan government spokesman Jordi Turull—himself under investigation on criminal charges of contempt of court, abuse of authority and misuse of public funds—said the administration's "absolute" policing priority is terrorism, not stopping the referendum.
"The fight against terrorism is the priority when it comes to mobilising Catalan and European police officers."
"The Catalan government sets priorities as far as the Mossos are concerned", he said: "the priority is stopping attacks and catching terrorists".