Meanwhile, Spanish Prime Minister Mariano Rajoy will miss an European Union summit in order to deal with the referendum, a government source said.
The Spanish Interior Ministry in Madrid said on Tuesday the order to note down the personal details of officials was given to police by the top state prosecutor in Catalonia.
Catalonia, a wealthy region of 7.5 million people in north-eastern Spain which has Barcelona as its capital, has its own language and culture but is not recognised as a separate nation by the Spanish state.
All of these declarations come a day after the Catalan regional police (Mossos d'Esquadra) received the order from the Catalan Prosecutor's office to seal off schools which have been designated as polling stations. The Catalan government is expected to hold snap elections if it loses the vote, which could imperil separatists parties' majority in the regional parliament.
It is cracking down on organizers by threatening them with prosecution. "I think it would be foolish not to, if you're talking about staying with a truly great, attractive and very historic country".
Prime Minister Mariano Rajoy has said the referendum is against the law and the constitutional court has ordered it be halted while its legality is determined.More news: GOP Leaders Pull Latest Health Care Repeal Effort
More news: Saudi allows women to drive
More news: Rhode Island gas prices down 3 cents per gallon
Rajoy's government claims the referendum would violate the country's constitution.
The government has in recent weeks taken political and legal measures to prevent the referendum by exerting more control over the use of public funds in Catalonia, arresting regional officials and seizing election leaflets.
This has sparked protests in Barcelona and other cities.
"I bet you if you had accurate numbers and accurate polling, you'd find that they love their country, they love Spain and they wouldn't leave, so I'm just for a united Spain", Trump said.
Pro-independence campaigners have defied Spanish authorities by handing out millions of ballots ahead of Sunday's vote.
But the unrelenting opposition from Madrid means such a result would go all but unrecognised, potentially setting up a new phase of the dispute. He also voiced his worry that Sunday could see violence in the Catalan region. "They are excluding half the population", Dastis said.