The Catalan regional government claimed the move threatened freedom of speech in the EU state as Spanish authorities’s attempts to crush the independence vote intensified.
A regional government spokesman said: “What they’re doing by blocking domain name servers is doing what Turkey does and what China does and what North Korea does.”
“No western democracy does that. The internet is the kingdom of freedom.”
High court judge Mercedes Armas said the regional authorities have continued to use websites to promote the referendum despite a ban on such activities.
Catalan president Charles Puigdemot shared links to websites with information on how to vote, but a court order saw the websites blocked in a move that angered the regional government.
Pro-independence group Catalan National Assembly (ANC) had their website was taken down by police before being move to another address.
The ANC said: “We received no notification whatsoever from the Guardia Civil.”
“It shows that the government cares more about stopping people from voting than they do about preserving freedoms in Spain.”
The move comes as the autonomous region’s police force, Mossos, expressed reservations over orders to close polling stations for Sunday’s vote.
The force warned public order would be at risk if they move to shut polling stations.
Mossos has already been accused of split loyalties with Spain bringing in thousands of extra national police officers ahead of the referendum.
Madrid has backed its dramatic measures by branding the referendum illegal, claiming it violates the country’s constitution.
Police questioned 17 people on suspicion of “disseminating a website for people to participate in a referendum declared illegal by the constitutional court”.
Officers have been ordered to seize national referendum materials and block people from casting their votes in public buildings.
Millions of ballot papers were seized and 14 officials from the Catalan government arrested as Spain scrambled to stop the “illegal” election.
The Catalan government has sent a letter of protest to the European Union.
The regional authority claimed blocking websites was part of “the ongoing unlawful repression of the institutions of autonomy of Catalonia” in the letter.