The occupations were partly organized by a platform of "schools open for the referendum" which distributed instructions on how to occupy buildings and keep them open as polling stations.
With less than 36 hours until polls are due to open in the Catalan independence referendum on Sunday, neither the Spanish government, nor the Catalan regional authority shows any signs of backing down from their postures.
Tens of thousands of Catalans are expected to vote in the ballot, which will have no legal status as it has been blocked by Spain's constitutional court.
This followed an order by Catalonia's High Court yesterday for police to prevent electronic voting taking place. Google has also deleted a smartphone application publicized by regional Catalan officials aimed at directing voters to their nearest polling stations. "We would lose everything, in addition to the unity of Spain", he said.
From district to district, people gathered to form "Committees to protect the referendum", using the Telegram messaging app to get organised and urging everyone to remain peaceful, said an AFP correspondent who saw some of the messages.
The Infrastructure Ministry announced on Friday that the airspace over Barcelona would be closed to helicopters and light aircraft until Monday.
How the 17,000 Catalan regional police respond to this order is regarded as key to the success or failure of the planned vote.
La Liga's threat to expel Barca in the event of independence would leave the club, estimated by Forbes magazine this year to be worth $3.64 billion, and its players, including five-times world player of the year Lionel Messi, without a league.
The referendum was not authorized by the federal authorities and Spain's Constitutional Court has ruled that it is illegal.
Earlier in the day, Spain's Civil Guard entered Catalonia's Center for Telecommunication and Information Technology (CTTI) to switch off the software, necessary to count electronic votes at the independence referendum.
The raid was meant to stop the use of vote-counting software linked to Sunday's referendum, Piqué said, adding that the Catalan government has an alternative to the software. The Spanish government has fought the measure and declared the vote illegitimate.
But Spanish unionists argue Catalonia already enjoys broad autonomy within Spain, along with other regions like the Basque Country and Galicia.
"This is absolutely a matter for prosecutors and judges", one high-ranking Spanish government official said on Thursday.
"The topic tires us, we want this dispute to end because we no longer talk about the problems of the rest of the country".
Public support for the referendum within Catalonia, a wealthy region in Spain's northeast, has become increasingly vocal as the vote has neared.