A dozen Catalan separatists will go on trial on February 12 over their bid to make the region independent from Spain, the Supreme Court has said, naming former prime minister Mariano Rajoy among those called to testify.

Nine of the accused are charged with rebellion for their role in the dramatic breakaway attempt in October 2017. They face up to 25 years in jail.

Rajoy, who ruled Spain until last June, will be one of "hundreds" of witnesses to testify at the trial, a Supreme Court document said.

He sacked Catalonia's regional government and called snap local elections after the Catalan parliament declared independence in October 2017 following a banned independence referendum.

Rajoy was unseated by a no-confidence vote backed by Catalan separatist parties last year.

The trial will last "around three months", with a verdict expected within three months, the president of Spain's Supreme Court, Carlos Lesmes, told reporters in Madrid.

"This is the most important trial which we have known in democracy," he added.

The trial will be broadcast in full on Spanish TV and it is is expected to have national political implications.

Tensions are likely to flare up between the central government and the separatists who rule Catalonia despite Prime Minister Pedro Sanchez's attempts to defuse the independence issue.

That makes it unlikely Catalan separatist parties will give their crucial backing for his minority government's 2019 budget, said Eurasia Group analyst Federico Santi.

"Failure to pass the budget, in turn, is likely to prompt Sanchez to resign and bring the 2020 elections forward to later this year," he added.

Rajoy's deputy prime minister Soraya Saez de Santamaria and Barcelona's current mayor Ada Colau are among those summoned to testify at the trial.

The court said it would also hear testimony from police officers and citizens who were injured during clashes during the October 1, 2017 independence referendum.

Riot police beat protesters and stormed polling stations on the day of the referendum in an attempt to stop it from going ahead.

In pre-trial detention for months, the nine jailed defendants were being transferred from the northeastern region of Catalonia to prisons near Madrid on Friday.

Supporters in Barcelona cheered them as they were taken away, waving the region's red, yellow and blue separatist flag and holding up umbrellas to guard against the rain. Some held up black and white signs in English that read "Fake justice!".

Three others will also stand trial, accused of disobedience and misuse of public funds.

Catalan President Quim Torra read a statement in English and Catalan in support of the defendants, saying "they have not committed any crimes".

Supporters of independence for Catalonia call the trial a "farce". But many Spaniards support it, having looked on in disbelief as the rich region's then executive tried to secede.

Catalonia's president at the time of the secession bid, Carles Puigdemont, is also accused of rebellion.

But he fled Spain after the region's majority-separatist parliament made a short-lived declaration of independence on October 27, 2017 which triggered Spain's worst political crisis in decades.

He is not among the 12 going on trial this month in Spain, which does not try suspects in absentia.

The trial's main protagonist is former Catalan vice president Oriol Junqueras, who opted to remain in Spain. He faces up to 25 years in jail on charges of rebellion and misuse of public funds.

Puigdemont and fellow separatists have all denied any wrongdoing, saying the 2017 referendum gave them the authority to break away from Spain.

The Catalan government claimed some 2.3 million people took part in the referendum and that 90 per cent had backed independence. Those figures could not be independently confirmed.

Far from curbing the desire for independence, the prosecution of the separatist leaders has energised the movement for Catalan self-determination and helped to keep the movement united despite deep divisions over strategy.

Hundreds of Catalan separatists protested outside an EU office in Barcelona on Friday night against what they claim is the unfair treatment of the defendants.

They hung a banner from the office's balcony that read in English: "Self-determination is a right not a crime".

Twenty-one protesters entered the office and plan to spend the night, a spokeswoman for influential grassroots separatist group ANC which organised the demonstration told AFP.

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