Spain is bracing itself for an unprecedented challenge to its territorial unity as the Catalan regional government stages an independence referendum that has been suspended by the country’s constitutional court and dealt a series of devastating blows by the central government in Madrid.

The pro-sovereignty administration of Catalan president Carles Puigdemont says that as many as 5.3 million people are eligible to vote in the unilateral poll and has vowed to declare independence within 48 hours of a victory for the yes campaign. But the Spanish authorities, which have ruled Sunday’s referendum illegal and unconstitutional, insist that the vote will not take place.

After a tumultuous 10 days that have seen Catalan government officials arrested, referendum websites blocked and millions of ballot papers seized, the Spanish government said it was confident it had dismantled the electoral apparatus.

On Saturday, Enric Millo, Madrid’s most senior representative in Catalonia, announced that police had sealed off 1,300 of the region’s 2,315 polling stations, while Guardia Civil officers acting on a judge’s orders had searched the HQ of the Catalan technology and communications centre, disabling the software connecting polling stations and shutting down online voting applications.

“These last-minute operations have allowed us to very definitively break up any possibility of the Catalan government delivering what it promised: a binding, effective referendum with legal guarantees,” he said.

“That’s what the Catalan government had promised to deliver on 1 October. Today, we can assure people that it will not go ahead.”

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Millo also said the Catalan regional police force, the Mossos d’Esquadra, would clear schools intended to serve as polling stations by 6am on Sunday. According to police, 163 schools across Catalonia have been occupied by parents and students who are protesting at what they see as a heavy-handed response by the Spanish authorities.

Millo said the government was not anticipating any trouble on Sunday morning, adding that he trusted in the “high level of common sense of the Catalans”. He added: “Everyone has the right to express themselves and to protest peacefully, but people also have to respect the orders of a judge.”

Joan García, a 39-year-old agricultural engineer, was one of the two dozen people who spent Friday night at the Cervantes primary school in central Barcelona. “Like different mums and dads, given what’s happening, we had a meeting on Friday to see if we could do anything, and 20 or 30 people from the school and the neighbourhood slept here last night,” he told the Guardian.

“We’ve been playing card games, basketball, doing some painting and making music with tambourines.”

Facebook Twitter Pinterest Pro-Spanish unity demonstrators march against the referendum in Barcelona on Saturday. Photograph: Enric Marti/AP

García said the group was planning to stay in the school for the whole weekend. “It’s a way to show we are against what the Spanish state is doing and against the police repression. We’re going to carry on with out activities all the way through Sunday. We’re not afraid. This is peaceful resistance. If they kick us out, we’ll sit on the street. It’s their problem.”

The Catalan National Assembly, which has coordinated the region’s drive for independence, admitted on Saturday that recent actions by the police and courts could affect voter turnout. But its president, Jordi Sànchez, said that a turnout of 1 million voters – less than a fifth of the electorate – would still make the referendum an “overwhelming success”.

Saturday afternoon saw a rare show of unionist strength in Barcelona as thousands of people marched through the centre of the city to call for the region to remain part of Spain. Another pro-unity rally took place in central Madrid, accompanied by Spanish flags and cries of “Viva España!”

At a closing rally for the independence campaign in Barcelona on Friday, people formed the slogan “Referendum is democracy” in large white letters in front of a cheering crowd, with many draped in the red and yellow Catalan flag.

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Tensions have risen across the country as the vote has neared. Puigdemont has accused the Spanish authorities of exceeding “the limits of a respectable democracy” and dragging Spain back to the Franco era. But despite the crackdowns, he remains adamant that the vote will happen.

On Saturday, Spain’s foreign minister, Alfonso Dastis, dismissed the Catalan government’s plans, telling the Associated Press they were “a mockery of democracy; a travesty of democracy”.

Although an overwhelming majority of Catalans want to have a referendum on sovereignty, polls suggest that slightly more are in favour of remaining part of Spain. The issue of independence is deeply divisive in the region, and some have accused the Catalan government of blatantly ignoring the court rulings that are thwarting the push for sovereignty.

Luis Rodríguez Vega, president of the Professional Judges’ Association of Catalonia, said judges were coming under intense political and public pressure as they tried to do their duty. “The conflict in Catalonia is coming from the government and the parliament, and that’s what’s so unprecedented: the institutions aren’t respecting the rules that lend them legitimacy,” he told the Guardian.

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“The government has decided that its legitimacy comes from the people and that it can ignore the constitution … As judges, we’re feeling an atmosphere of pressure but we’re not scared. We’ve come to a point where democracy has been usurped: if you oppose this whole movement, you’re called a fascist.

“Are we mad? I’m trying to obey the law, and there are a minority of Catalans who don’t want to vote, OK? I have to defend that minority and that means the laws must be followed.”

Puigdemont and Ada Colau, the mayor of Barcelona, have called on the EU to step in and mediate, but it says the issue is an internal Spanish matter. “[It’s] a Spanish problem in which we can do little,” Antonio Tajani, the president of the European parliament, said on Friday.

However, Tajani said that although the EU was backing the Spanish government – because “on a legal level, Madrid is right” – there would need to be political discussions the day after the vote.