Officers must close and cordon off the centres until 9 p.m. on Sunday evening and make it clear that the police cordon is not to be broken, warning people they are liable to criminal prosecution if they do so.
They have been ordered to remove anyone already in the polling stations, and seize any items being used for the vote, "especially ballot boxes, computers, ballot papers and election documents and propaganda".
They may repeat the removals and seizures if people try to break the police line during the day.
The cordons and closures must be in place by Saturday, September 30.
Mossos officers must report any infractions of the polling station orders, and may request local police, National Police and Civil Guard back up if they believe they need it.
The Catalan Police must also prevent people designated as polling officers from being able to access the premises designated as polling stations, which is due to happen from 7:30 a.m. on Sunday.
Finally, the Mossos have been told to prevent the vote taking place anywhere within 100 metres of the designated premises, "including on public streets".
Catalan Police chiefs are worried about the public order and manpower requirements stemming from the order, Spanish media reported.
The prosecutor says that despite efforts by the central government to disable the logistical preparation for the vote, Catalan leaders are insisting some kind of vote will take place on Sunday.
Also on Wednesday morning, the Catalan First Minister unilaterally called a meeting of the Catalan Security Council, a regional policing body. That move was rejected by the Home Office two days ago.
A meeting of senior police chiefs in the region is taking place this morning. The prosecutor's office in Catalonia told The Spain Report that the polling station issue would likely be on the agenda.