More than 160 schools in Catalonia have been occupied by activists trying to keep them open ahead of the region's banned independence referendum, Spain's central government says.
Police visited 1,300 of the 2,315 schools in Catalonia designated as polling stations, finding 163 occupied.
Tens of thousands of people are expected to attempt to vote on Sunday.
Demonstrators also rallied in Barcelona on Saturday evening against the poll, calling instead for unity with Spain.
Waving Spanish flags and carrying banners reading "Catalonia is Spain", thousands marched on the town hall.
Meanwhile, Spanish authorities are stepping up their attempts to stop the ballot taking place.
Authorities in Madrid have sent thousands of police to the region to stop the referendum - declared illegal by Spain's constitutional court. The Madrid government has ordered the Catalan regional police force, the Mossos d'Esquadra, to assist them.
Many of the extra officers drafted in are stationed in two ships in the port of Barcelona.
Police have also occupied the regional government's telecommunications centre.
Police have also been ordered to clear schools occupied by activists aiming to ensure the buildings can be used for voting.
Spanish government sources quoted by Reuters said police would decide for themselves how to enforce orders to stop people voting. The head of the Catalan police has urged officers to avoid using force.
Many of those inside the schools are parents and their children, who remained in the buildings after the end of lessons on Friday. Some told Reuters news agency that police had told them they could stay as long as they were not doing anything connected to Sunday's vote.
Laia, a 41-year-old sociologist who is staying in a Barcelona school this evening, said the police had visited four times.
"They read us out the part of the court order that says no activities related to the preparation of the banned referendum are allowed," she told Reuters.
What is the basis for the vote?
Catalonia, a wealthy region of 7.5 million people in north-eastern Spain, has its own language and culture.
It also has a high degree of autonomy, but is not recognised as a separate nation under the Spanish constitution.
Pressure for a vote on self-determination has grown over the past five years as austerity has hit the Spanish economy and people hard.
But Spanish unionists argue Catalonia already enjoys broad autonomy within Spain, along with other regions like the Basque Country and Galicia.
Will the vote go ahead?
Officers have been visiting the locations due to be used as polling stations, as well as seizing items such as ballot papers, while prosecutors have ordered the closure of websites linked to the vote and the arrest of officials organising the referendum.
But Catalan leader Carles Puigdemont told Reuters that the referendum would go ahead.
"Everything is prepared at the more than 2,000 voting points so they have ballot boxes and voting slips, and have everything people need to express their opinion," he said.
Why is Madrid so opposed?
Spanish Prime Minister Mariano Rajoy stared down Catalan secessionists when they held a trial referendum in 2014, offering no concessions to their demand for a legal vote.
He has pledged to stop the 2017 vote, saying it goes against the constitution which refers to "the indissoluble unity of the Spanish Nation, the common and indivisible homeland of all Spaniards".
Central government spokesman Iñigo Mendez de Vigo accused the Catalan government of being inflexible and one-sided but it is a charge Catalan nationalists throw back at Madrid itself.
Despite the tension in the region, demonstrations by independence campaigners have been largely peaceful.
"I don't believe there will be anyone who will use violence or who will want to provoke violence that will tarnish the irreproachable image of the Catalan independence movement as pacifist," Mr Puigdemont said.