MADRID (AP) — Spain's government on Tuesday ordered Catalonia's regional police force to identify officials at local schools and other civic buildings expected to be used as polling stations in a controversial independence referendum.

Spanish authorities say the Oct. 1 ballot on whether the Catalan region should secede from Spain is illegal. It is cracking down on organizers by threatening them with prosecution.

The order placed further pressure on the Catalan police, who were being told by central Spanish authorities to act against their fellow Catalans in the emotionally-charged standoff.

The Spanish Interior Ministry in Madrid said the order to note down the personal details of officials was given to police by the top state prosecutor in Catalonia.

The prosecutor also instructed the Catalan police, known as the Mossos d'Esquadra, to commandeer the polling stations ahead of the weekend, Spanish news agency Europa Press reported.

As tensions surrounding the vote builds, Prime Minister Mariano Rajoy has canceled his trip to the Estonian capital of Tallinn for a European Union summit on Friday, Spanish media reported.