The Guardia Urbana force was obeying an order from prosecutors to prevent the Catalan vote going ahead on Sunday.

Spain is deploying thousands of extra police in the region.

About 10 million ballot papers have been impounded, and websites informing Catalans about the referendum have been shut down.

Guardia Urbana sources told the Catalan daily La Vanguardia on Thursday that the police would stop anyone trying to move ballot boxes to another location.

The Catalan President, Carles Puigdemont, has told Catalans on Twitter how they can find a polling station to cast their votes - by using a special app.

Meanwhile, Barcelona Mayor Ada Colau has appealed to the European Commission to mediate in the standoff between Catalonia and the Madrid government, in an opinion piece in The Guardian.

She also accused Madrid of having allowed the long-running Catalan sovereignty issue "to escalate from an internal dispute to a European conflict".

Catalonia, a wealthy region of 7.5 million people in north-eastern Spain, has its own language and culture, and has a high degree of autonomy. But it is not recognised as a separate nation by the Spanish state.

Spain has chartered three cruise ships to house up to 6,000 extra police officers in Barcelona and another port city, Tarragona.

But in a defiant interview, Mr Puigdemont said it was impossible for Spain to "seal off" the region.

Mr Puigdemont told the Spanish daily El Diario that "we're not going to advise anyone to do anything violent" despite the Spanish pressure to block Sunday's vote.

The Catalan police force - the Mossos d'Esquadra - is about 16,800 strong and has been ordered by Catalonia's chief prosecutor to help block the referendum preparations.

But the Mossos has warned that public order could deteriorate if its officers are deployed to prevent people from voting.

Spain's paramilitary Guardia Civil force has beefed up its presence in Catalonia.

Mr Puigdemont insisted that holding the referendum "is not a crime... and there is no threat to security".

"Clearly the Spanish government has a warlike determination to prevent the referendum," he said.

But he also said "a unilateral declaration of independence is not on the table" - even if the pro-independence Catalan government got the Yes vote it desired.

"What is on the table now is just a roadmap: and the referendum is Day One," he said, urging Madrid again to open direct talks with Catalonia on the region's independence bid.