The proceedings, which begin Tuesday, will be broadcast live on television and all eyes will be focused on the impartiality of the Spanish Supreme Court.
Catalonia's separatists have attacked the court's credibility in the run-up to the trial, saying it is a puppet of the Spanish government and any ruling will be a political one that has been decided in advance.
"In reality, it's democracy itself that will go on trial," Oriol Junqueras, one of the accused, wrote from jail in reply to questions sent by The Associated Press. "We are before a trial which, through a partial investigation full of falsities and irregularities, criminalizes a political option and an ideology."
But Supreme Court president Carlos Lesmes dismisses that notion, saying the trial is the most important since Spain's transition to democracy in 1977 after the death of dictator Gen. Francisco Franco.
"This is a trial following the highest standards set by the European Union," Lesmes recently told a group of journalists.
Lesmes says the outcome of the trial will reverberate beyond the political crisis in Catalonia, while recognizing that the Supreme Court's integrity is at stake.
"I certainly believe that there is a huge campaign to discredit the Spanish judiciary, which forms part of a defense strategy," he said.
Spanish authorities say that the separatists are guaranteed a fair trial by the very democracy founded on the rule of law that they allegedly violated.
Lesmes rejected the idea that Spanish courts operate at the whim of the government, pointing to recent guilty verdicts for prominent members of the political and economic elite, including last year's graft conviction of former members of Rajoy's then-ruling party and the imprisonment, also on graft, of the king's brother-in-law.
Junqueras, the former vice president of the Catalan regional government, and 11 others are being tried for their roles in holding an independence referendum on Oct. 1, 2017, after ignoring a ban by the country's Constitutional Court, and for the subsequent declaration of independence 26 days later despite more warnings from authorities.
The conflict with Catalonia has been festering ever since, with a regional election on Dec. 21, 2017, showing that the 7.5 million residents of Catalonia remain divided by the secession question.
Junqueras himself faces the largest possible sentence of 25 years for rebellion. He and eight other defendants have spent over a year in pre-trial custody because they were considered to be flight risks.
For Junqueras, the reasons are instead "revenge and exemplary punishment."
"This is a warning for future generations against questioning the state's status quo," he wrote, "even if that diminishes its own democracy."
Junqueras' boss, former Catalan president Carles Puigdemont, and seven associates fled Spain to other European countries and have succeeded in avoiding extradition.
Proceedings are likely to last for at least three months. The verdicts, and any sentences, would be delivered months later.
More than 500 witnesses have been called to testify in court, including former Prime Minister Mariano Rajoy, Catalan and central government officials, police officers and ordinary citizens.
The charge of rebellion will hinge on whether the prosecution can establish that the separatists employed violence during the breakaway attempt. They also face charges of sedition, which doesn't imply violence, disobedience, and the misuse of public funds.
"Here they are accusing our clients of a crime of rebellion when they didn't go out on the streets with tanks, or in uniforms or with weapons," Jordi Pina, the defense attorney for Turull and two other defendants, told the AP. "The only thing they did was to allow ordinary citizens who wanted to take a ballot and put it in a ballot box to do so."
Politically, the stakes are high. A harsh sentence would further alienate many Catalans, possibly even some who haven't been seduced by the idea of independence. The start of the trial coincides with Spanish Prime Minister Pedro Sanchez needing the support of Catalan separatist parties to pass his national budget.
Given that Europe will be closely observing the case, Sanchez paid a visit last week to the European Council and the European Court of Human Rights in Strasbourg, France. The case could eventually end up at that European court on appeal.
Without directly mentioning Catalonia, Sanchez told the European Council that Spain protects "the differences and peculiarities of every one of its regions" as opposed to "those who sustain political projects based on false fictions that incite hate and division."
Joseph Wilson reported from Barcelona.
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.