They also arrested 14 organizers of the vote, including several top Catalan government officials.
MADRID-Spain will send more state police to Catalonia to block any moves to hold a banned independence referendum, the interior ministry said on Friday.
"They will be tasked with surveillance of public space and maintaining order and they will act in case the illegal referendum is maintained", the interior ministry said in a statement.
"This is a very delicate situation for the Mossos because the prosecutors' orders are basically to repress your own citizens for exercising their democratic right to vote", said Josep Miquel Milagros, a spokesman for USPAC, a police union in Catalonia that represents about 2,600 officers.
Spain's Constitutional Court has suspended the October 1 vote while judges assess its legality, but Catalan authorities insist it will take place, and both sides have accused the other of acting illegally and undemocratically.
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Meanwhile, Ian Williams, the other contributor on the panel, said the Catalan separatists are playing very cleverly by provoking the government in Madrid, "because they know the government is stupid enough" to fall in the trap.
Visitors to Barcelona are being warned of demonstrations in the city that could disrupt travel.
The growing police presence and aggressive crackdowns are fueling mounting protests among the Catalan population, with thousands taking to the streets of Barcelona, and dock workers refusing to provide services to the police staying on the ferries.
Catalan National Assembly president Jordi Sanchez told the cheering crowds: "We will be here, peacefully but present, until all of the arrested walk out free".
Article 155 of the Spanish Constitution allows the central government to take control of a regional administration if it poses a threat to the national interest. Polls consistently show the region's inhabitants favor holding a referendum but are roughly evenly divided over independence from Spain.