MADRID (AP) — A sensitive trial against a dozen Catalan separatist politicians and activists got underway Tuesday in Spain's Supreme Court amid protests by pro-independence supporters and a highly 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.

Sanchez could be forced to call an early election if the Catalan separatists, whose support brought the Socialists to power last year, don't change their current 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 Sanchez to agree to talks on self-determination for their region, but the government argues that the country's constitution doesn't allow it.

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 Spain's Supreme Court in Madrid.

Former Catalan Vice President Oriol Junqueras, the regional parliament's former Speaker Carme Forcadell and the other 10 defendants weren't expected to testify on Tuesday, but they sat on four benches in the middle of the courtroom.

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

Junqueras' lawyer, Andreu Van Den Eynde, was the first to speak, arguing that the cause goes "against political dissidence."

"We are before an exceptional trial," he told the judges, adding that "self-determination is the formula to avoid conflicts in the world."

Catalan President Quim Torra, a fervent separatist, followed the 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.

Junqueras faces up to 25 in prison if he's found guilty of rebellion, while others charged with sedition or misuse of public funds could get lower sentences if convicted.

The proceedings were 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 were likely to last for at least three months. The verdicts, and any sentences, will be delivered months later.

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Joseph Wilson reported from Barcelona.