A sensitive trial against a dozen Catalan separatist politicians and activists has got under way at Spain’s Supreme Court amid protests by pro-independence supporters and a volatile political environment.

The defendants are being tried on rebellion and other charges stemming from their roles in pushing ahead with a unilateral independence declaration in October 2017.

The declaration was based on the results of a divisive secession referendum that ignored a constitutional ban.

The trial, arguably Spain’s most important in four decades of democracy, began as the future of prime minister Pedro Sanchez’s minority government hinges on last-minute negotiations with Catalan pro-independence parties to back his 2019 budget.

Pedro Sanchez
Pedro Sanchez (Andrea Comas/AP)

Mr Sanchez could be forced to call an early election if the Catalan separatists, whose support brought the Socialists to power last year, do not change their position of voting against the prime minister’s spending plan on Wednesday. A debate in the parliament’s lower house began Tuesday.

The separatists want him to agree to talks on self-determination for their region, but the government argues that the country’s constitution does not allow it.

Opening the parliamentary debate, budget minister Maria Jesus Montero told Catalan legislators the government would “not give in to any blackmail by anybody”.

“Under no circumstance will we admit that the right to self-determination in Catalonia appears in any talking points,” she said.

The defendants face the Supreme Court in Madrid
The defendants face the Supreme Court in Madrid (Emilio Naranjo/AP)

Mr Sanchez appeared to put more pressure on his opponents by tweeting that “the right-wing and the separatists will vote against a budget that helps social causes”.

“They both want the same thing: a Catalonia that is divided and a Spain that is divided,” he wrote.

Tensions between regional and central authorities peaked with the 2017 breakaway attempt but the conflict has been festering ever since. The 7.5 million residents of Catalonia remain divided by the secession question.

On Tuesday, pro-independence protesters briefly blocked highways and roads before the trial began at the Supreme Court in Madrid.

Former Catalan vice president Oriol Junqueras, the regional parliament’s former speaker Carme Forcadell and the other 10 defendants are not expected to give evidence on Tuesday, but they sat on four benches in the middle of the courtroom.

They faced a seven-judge panel headed by Supreme Court magistrate Manuel Marchena, who moderates proceedings.

Junqueras’s lawyer, Andreu Van Den Eynde, was the first to speak, arguing that the case goes “against political dissidence”.

“We are before an exceptional trial,” he told the judges, adding: “Self-determination is the formula to avoid conflicts in the world.”

Oriol Junqueras, left, speaks with his lawyer
Oriol Junqueras, left, speaks with his lawyer (Emilio Naranjo/AP)

Catalan president Quim Torra followed proceedings from the back of the courtroom where 100 seats were reserved for defendants’ relatives, journalists and members of the public who lined up for hours to get one of the limited spots.

Carles Puigdemont, Mr Torra’s predecessor who fled from Spain, dubbed the trial “a stress test for the Spanish democracy”.

Addressing reporters at a news conference in Berlin, he added: “I trust, however, that the Spanish state will take advantage of this chance to issue the correct sentence, which is absolution.”

Puigdemont successfully avoided extradition to Spain when a German court refused to send him back on charges of rebellion last year. Those who stayed behind and showed up in court are the ones facing trial now.

Carles Puigdemont at a news conference in Berlin
Carles Puigdemont at a news conference in Berlin (Markus Schreiber/AP)

His number two at the time, Junqueras, faces up to 25 years in prison if he is found guilty of rebellion, while others charged with sedition or misuse of public funds could get lower sentences if convicted.

The proceedings are being broadcast live on television in a display of transparency that aims to fight the separatists’ attack on the court’s credibility.

Authorities in Spain have dismissed the notion that the trial is political and say it follows the Europe Union’s highest standards.

Proceedings are likely to last for at least three months. The verdicts and any sentences will be delivered months later.