The trial of 12 Catalan separatist leaders in Madrid is of “paramount importance to Spain’s reputation as a modern democracy”, the Spanish ambassador to London, Carlos Bastarreche, has said in a rare public intervention intended to convince the world the trial is not political.

The ambassador, acting as part of a Europe-wide effort by the Madrid government, accused the pro-independence Catalan regional government of “a massive campaign of disinformation” and said the best way to fight back was with transparency.

The trial of the 12, nine of whom are accused of rebellion, begins on Tuesday. Those accused of the most serious charges could face up to 25 years in jail.

The intervention shows that the Spanish government is nervous the high-profile trial could turn the European public against it by seeking to use judicial methods to repress Catalan demands for independence.

Officials from the highly active Catalan foreign ministry have already been to the UK to speak to MPs and claim that the proceedings, which could last as long as three months, amount to a political show trial.

The Madrid government has launched a campaign entitled This is the Real Spain, highlighting the country’s diversity, to counter the Catalan independence movement’s claim that a politicised judiciary is repressing political opinion.

Bastarreche said: “There is a real concern of the Spanish government because the image of Spain can be negatively effected by the campaign of disinformation, and it has been decided to put in place a response.

“We are facing a very organised information campaign that is based on spreading fake news. The principal underlying message of our opponents is that Spain is not a democracy.”

The government is ensuring the trial is streamed live and is publishing factsheets about the independence of the Spanish judicial system to try to counter the Catalan narrative.

Q&A What can we expect from the Catalan independence leaders' trial? Show The proceedings, which began on 12 February, are presided over by seven supreme court judges, and are expected to last around three months. The court will sit for three days a week – Tuesdays to Thursdays – from 10am to 6pm. Hundreds of witnesses are to be called, among them the former Spanish prime minister, Mariano Rajoy, his former deputy, Soraya Sáez de Santamaría, the mayor of Barcelona, Ada Colau, the former Catalan president Artur Mas and the current speaker of the Catalan parliament, Roger Torrent. The court has ruled that Carles Puigdemont, the former Catalan president who led the push for independence before fleeing into self-imposed exile, will not be allowed to testify via videolink from Belgium. Other witnesses will include some of the Catalan voters and Spanish police officers injured on the day of the referendum. The verdict in the case, which is likely to be delivered in June, could be appealed by the defendants in Spain’s constitutional court. The next step after that would be an appeal to the European court of human rights in Strasbourg. Sam Jones in Madrid

Bastarreche acknowledged Spain had a unique challenge in explaining its case in the UK, where the Westminster parliament had permitted Scotland to hold an independence referendum. No such referendum has been granted by the Spanish parliament.

He said: “There is a special difficulty to understand in this country what is happening in Spain. Britain is different and does not have a written constitution and no enshrinement of the unity of the country. That is a product of our history. The indissolubility of our country is one of the main principles of our country.”

He pointed to the demonstration in Madrid on Sunday objecting to the government’s handling of the Catalan crisis as proof that diversity flourished in his country, just as Brexit had shown the range of opinion in the UK.

“Spain is one of the most solid democracies in the world, including one of the most protective judicial system as far as the defendants’ rights are concerned,” he said.

A Catalan separatist narrative – pointing to lengthy detentions without trial, the heavy potential sentences, and the quality of the evidence that any of the accused agitated for violence – has largely shaped European opinion over the last year.

Alfred Bosch, the Catalan foreign minister, said the Spanish government campaign showed that Madrid “feels fragile and uneasy. These trials will not defuse a crisis that needs to be defused by politics.

“If the sentences are heavy – and the charges are heavy – it will create uproar in Catalonia, Spain and Europe. We are not acting on disinformation.

“It is quite obvious in the 21st century in Europe either people decide on their own future, or there will be no permanent solution. The only violence that occurred during the referendum on 1 October was committed by the Spanish police, or Guardia Civil, and not by the Catalan government.”

Bosch claimed the lead judge in this case was a nationalist, and pointed to the way European courts have rejected Spanish government extradition requests for the Catalan leaders who fled the country after the Spanish government ruled the referendum unlawful.

“The accused are not charged with stealing. They are charged as an autonomous government with holding an illegal referendum. This will affect the reputation of Spain for years to come. Ideas cannot be prosecuted,” he said.

The pro-independence government of the then Catalan president, Carles Puigdemont, pressed ahead with the unilateral referendum in autumn 2017 even though polls consistently show Catalonia is evenly split over the independence issue, and pro-independence parties have never managed to win 50% of the vote in the regional parliament.

According to the Catalan government, about 2.3 million of Catalonia’s 5.3 million registered voters – 43% – took part in the referendum, and about 90% of participants backed independence. The vote was largely boycotted by unionist Catalans.