Spain's crackdown on Catalonia's controversial independence referendum is only boosting support for the vote, the regional president has said.

Vowing to turn the domestic dispute into a European affair, Catalan leader Carles Puigdemont accused the European Commission of having “deaf ears” so far to Catalonia's desire to hold a poll.

Mr Puigdemont criticised the Commission for failing to defend Catalans against controversial measures adopted by the Spanish courts and central authorities to try to stop the vote.

No country, within or outside the European Union, has openly expressed support for the 1 October referendum that Spain's conservative government sees as illegal.

“They are very brave when they talk about other countries where they have no competencies, but where are they when we citizens need them?” he said of EU officials in an interview four days before the secession referendum. “Is Europe's solution to Catalans to turn its back?”

“If the yes wins, I will make an appeal for the European community to become involved, because we will have won our right to be heard, something that hasn't happened until now because the European Commission has always turned a deaf ear (to Catalonia),” the 54-year-old separatist leader said.

Sunday's vote, Mr Puigdemont said, will allow Europe to “hear the voice of Catalonia in a very loud and strong way".

“Whoever doesn't want to hear our voice needs to see a political otolaryngologist,” Mr Puigdemont said, using the formal term for an ear, nose and throat specialist.

He added that if European Commission President Jean Claude-Juncker cannot grasp the determination in Catalonia, “then it's because this (European) project is in bad hands".

Spain's Constitutional Court, which has previously ruled that only central authorities can call such a vote and that all Spanish nationals should vote on sovereignty matters, has suspended the referendum.

The pro-independence regional government has so far ignored court rulings and vowed to go ahead with the vote. Officials of the executive and legislative branches are under investigation and a heavy police presence has been assembled in the northeastern region as the day for Catalans to cast ballots nears.

Madrid has launched an unprecedented crackdown to stop the referendum, including seizing paper ballots, removing referendum propaganda and ordering schools to be sealed off so they can't be used as voting stations.

“Today we are closer to a massive (turnout for the) referendum than we were one month ago,” he said, describing the crackdown measures as “apocalyptic".

On Wednesday, Spain's National Court said it planned to investigate possible sedition charges for demonstrators who took part in a massive protest last week against a police crackdown on preparations for the vote.

“Looking at this landscape, somebody could think that we are hoarding weapons of mass destruction in Catalonia, a nuclear arsenal or a world-class drug stash,” Mr Puigdemont said. “But in fact, we are just trying to hold a referendum.”

He said that people in Catalonia that saw the referendum with indifference or hostility have now “seen the curtailing of freedoms as an offence to their democratic convictions".

Regional leaders have said that if the “yes” side wins, they would be ready to declare Catalonia's independence two days later regardless of voter turnout. But Mr Puigdemont on Wednesday acknowledged that significant participation is needed to portray the vote as representative.

He refused to disclose what percentage of the 5.5 million Catalan voters his government needs to declare the vote valid, but cited previous referendums in Spain, including the 2005 vote to pass the European Constitution that had a turnout of 42 per cent.

“Nobody raised concerns about the participation level in order to legitimate the results, so I hope there are also no concerns in this case,” Mr Puigdemont said.

Catalonia is one of Spain's 17 autonomous regions. Its capital is the Mediterranean port city of Barcelona.

With a population of 7.5 million inhabitants, its own cultural traditions and language, Catalonia contributes a fifth of the Spain's 1.1 trillion-euro economy (£877bn).

Catalonia referendum protests – in pictures Show all 10 1 /10 Catalonia referendum protests – in pictures Catalonia referendum protests – in pictures Demonstrators block a Guardia Civil vehicle as they try to leave the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures Demonstrators react as they try to stop the car carrying Xavier Puig, a senior at the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office, after he was arrested by Guardia Civil officers in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures A demonstrator reacts as he tries with others to stop the car carrying Xavier Puig, a senior at the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office, after he was arrested by Guardia Civil officers in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures Spokeswoman of the Catalan pro-independence anticapitalist party "Candidatura d'Unitat Popular - CUP" (Popular Unity Candidacy), Ana Gabriel, talks to the media in Barcelona Josep Lago/AFP Catalonia referendum protests – in pictures Republican Left of Catalonia party's (ERC) Member of Parliament Joan Tarda (C) attends a demonstration outside the regional Economy Ministry in Catalonia during a police search for documents connected with the organisation of the Catalan independence referendum, in Barcelona EPA/Alejandro Garcia Catalonia referendum protests – in pictures A man holds pro-referendum poster next to a Spanish Civil Guard who stands in front of the Economy headquarters of Catalonia's regional government in Barcelona. The operation comes amid mounting tensions as Catalan leaders press ahead with preparations for an independence referendum on October 1 despite Madrid's ban and a court ruling deeming it illegal Josep Lago/AFP Catalonia referendum protests – in pictures People hold placards reading "Democracy" as they protest in front of the Economy headquarters of Catalonia's regional government in Barcelona AFP Catalonia referendum protests – in pictures A crowd of protesters gather outside the Catalan region's economy ministry after junior economy minister Josep Maria Jove was arrested by Spanish police during a raid on several government offices, in Barcelona Reuters/Albert Gea Catalonia referendum protests – in pictures People holding 'Esteladas' (Catalan pro-independence flags) attend a protest near the Economy headquarters of Catalonia's regional government Lluis Gene/AFP Catalonia referendum protests – in pictures People demonstrate on a Spanish Civil Guard Police car outside the Catalan Vice-President and Economy office as police officers holds a searching operation inside David Ramos/Getty Images

The vast majority of Catalans favour holding a referendum, but they have long been almost evenly split over independence itself.

Separatist sentiment peaked at the height of the 2008 financial crisis, with many Catalans feeling they could do better on their own, but with the national and regional economies thriving again, polls indicate support for secession is on the wane.

The region's first attempts to hold a non-binding referendum in 2014 were blocked by Spain's Constitutional Court. The Catalan government went ahead and staged the unofficial poll. About 2.3 million Catalans — less than half of those eligible — voted, with 80 per cent favouring independence.