Spanish national police have sealed off 1,300 of 2,315 schools in Catalonia designated as polling stations for the banned independence referendum, the government in Madrid says.
The move came as the Spanish authorities stepped up their attempts to stop Sunday's referendum.
Police have now occupied the regional government's telecommunications centre.
Catalonia, a wealthy region of 7.5 million people in north-eastern Spain, has its own language and culture.
It has a high degree of autonomy, but is not recognised as a separate nation under the Spanish constitution.
Tens of thousands of people are expected to vote in Sunday's ballot, which has been declared illegal by Spain's Constitutional Court.
The authorities in Madrid have sent thousands of police to the region to stop it taking place.
They have been ordered to clear schools occupied by activists - including parents and their children who remained in the buildings after the end of lessons on Friday - aiming to ensure the buildings can be used for voting.
Officers from the Catalan regional police have been instructed to assist, but also told not to use force.
A government source quoted by Reuters news agency said 163 schools had been occupied by families.
Officers have also been 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.
Catalan leader Carles Puigdemont told Reuters news agency 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.
Friday saw thousands of Catalan separatists hold a final rally in the city of Barcelona.
But in the town of L'Hospitalet de Llobregat, some 2,000 people opposed to independence attended a rally called by Ciudadanos, Catalonia's main opposition party.
Friday also saw a judge order media giant Google to remove an application giving information about the outlawed referendum.
Pressure for a vote on self-determination has grown over the past five years but in the 2015 regional election, won by an alliance of pro-independence parties, about 40% of voters backed parties loyal to Spain.
Spanish unionists argue Catalonia already enjoys broad autonomy within Spain, along with other regions like the Basque Country and Galicia.