CATALAN president Quim Torra has issued a plea to the international community ahead of the trial by Spain of pro-independence leaders.

Spanish authorities transferred nine pro-independence politicians and activists from prisons in Catalonia to the capital Madrid ahead of a high-stakes trial later this month.

Catalans showed their support for the political prisoners in roadside demonstrations.

WATCH: Catalans show support as political prisoners moved to Madrid

The trial will begin on February 12 at the Supreme Court in Madrid, a court official said.

In addition to prosecutors and state attorneys, a far-right party that has recently emerged in Spanish politics sits on the prosecution bench.

Vox wants to use the trial to showcase its "defence" of Spanish unity ahead of European and local elections in May next year.

Torra said: “On the threshold of this trial against democracy, we call on the international community, citizens, governments, civil and human rights organisations, the media, and anyone committed to justice and freedom, to join the Catalan people in their defence of principles and values that make the world a place more fair, more safe and more free.

“The problem for the defendants in this trial, the problem for Catalan democrats, is also a problem for the international community, and especially the European Union.

“We are European citizens, with rights and freedoms that must be respected by Europe’s institutions.”

A panel of judges is due to issue a judicial order specifying which witnesses will be called and setting out a calendar for the trial, expected to last about three months, the official said.

Twelve defendants – nine in custody and three who were released on bail – could be imprisoned for decades if they are convicted of rebellion, the gravest of the charges.

They could also be fined if they are found guilty of misusing public funds. Defence lawyers say they should be acquitted.

The "trial of the century", as it has been labelled by domestic media, has taken on high political significance. Catalan pro-indendence figures have made clear they will use the proceedings to prove they are being tried for their ideas.

The proceedings will be public and televised.

The charges emerge from the dramatic autumn of 2017 that brought the deepest political crisis in decades to Spain, when Catalonia held an independence referendum on October 1 and a violent police crackdown attempted to stop it.

Catalan legislators made a unilateral independence declaration 26 days later after winning the vote.

Several more politicians, police officers and elected officials will also be tried in lower courts for their roles in the independence bid.

The Catalan leader at the time, Carles Puigdemont, is campaigning for Catalonia's independence from Belgium, where he has been forced to remain to avoid extradition.

Among those who remained behind and were put in custody are his former number two, ex-Catalan vice-president Oriol Junqueras, activist-turned-politician Jordi Sanchez, and the former speaker of Catalonia's regional parliament, Carme Forcadell.

Supporters gathered early at the gates of three prisons in Catalonia, where convoys of police emerged ferrying the nine defendants – seven men and two women.

Some activists tried to block the vehicles by throwing themselves on the road, but were quickly removed by police, television footage showed.

The nine defendants were grouped at Brians 2, a penitentiary 24 miles west of Barcelona, before being escorted by the Civil Guard to two prisons in Madrid.