A little less than 16 months ago, the nine men and three women sitting on the plush burgundy benches of Madrid’s supreme court were ministers in the Catalan regional government and high-profile civic leaders.
On Tuesday morning, however, they stepped out of prison vans to take their places as the defendants in landmark proceedings variously described as the trial of the century, a “stress test for Spanish democracy”, and the single most important judicial event the country has seen since the end of the Franco dictatorship.
The dozen are standing trial over their alleged roles in the regional independence crisis that pitched Spain into its worst political turmoil for four decades.
During the next three months, seven supreme court judges will investigate the roles the accused played in the run-up to the independence referendum in October 2017 and the subsequent unilateral declaration of independence.
Nine of the defendants – who include the former Catalan vice-president Oriol Junqueras, the former speaker of the Catalan parliament Carme Forcadell and two influential grassroots activists, Jordi Cuixart and Jordi Sànchez – are accused of rebellion, which carries a prison sentence of up to 25 years.
Other charges include sedition and the misuse of public funds.
The tone for the trial was quickly set by the heavy police and media presence, the protesters outside the court who greeted the arrival of Catalonia’s pro-independence president, Quim Torra, with cries of “Coup perpetrator!”, and by the first defence lawyer to speak.
Junqueras’ advocate, Andreu Van den Eynde, dismissed the case as a politically motivated exercise and told the court that prosecutors were trying to criminalise displays of free expression and curtail the defendants’ right to argue for self-determination.
“No international or EU law blocks the secession of a regional entity; self-determination is synonymous with peace and not war,” he said.
11 September 2017: an aerial view of people waving a giant banner saying ‘Independence now’ during a demonstration in Barcelona. Photograph: Roser Vilallonga/AFP/Getty Images
Van den Eynde, who is also representing the former Catalan foreign minister Raül Romeva, said his clients’ freedom of expression had been violated.
“The political arena is a free space,” he said. “Freedom of expression extends even to those ideas that shock and offend.”
He described the complicated legal process as a “procedural vaudeville” and said it was intended to put Catalan independence itself on trial.
Jordi Pina, a lawyer representing the Catalan MP and former civil society leader Jordi Sànchez, called for the court to allow testimony from King Felipe, arguing that the Spanish monarch’s controversial speech two days after the referendum had been cited in prosecution documents.
The king had accused Catalan authorities of attempting to break “the unity of Spain” and warned that their push for independence could risk the country’s social and economic stability.
Pina also urged the court to reconsider its decision not to allow testimony from the former Catalan president Carles Puigdemont via videolink from Brussels.
The trial – which began shortly after 10.20am on Tuesday and is being broadcast on television – will focus on the Puigdemont government’s decision to hold the referendum despite repeated warnings that it would violate the constitution, which stresses the “indissoluble unity of the Spanish nation”.
Although Catalan pro-independence parties have never managed to win 50% of the vote in the regional parliament, and although polls consistently show Catalonia is roughly evenly split over the independence issue, Puigdemont’s administration chose to press ahead with the vote.
Pro-independence parties managed to get a law paving the way for the referendum passed in the regional parliament in early September 2017, despite furious objections from opposition MPs, who complained that usual procedures had been disregarded.
The law was subsequently suspended and eventually struck down by Spain’s constitutional court.
But the Catalan government opted to hold the referendum, which was marred by violence. Spanish police officers raided polling stations, charged crowds with batons and fired rubber bullets in their attempts to stop the vote.
According to the Catalan government, about 2.3 million of Catalonia’s 5.3 million registered voters – 43% – took part in the referendum, and about 90% of participants backed independence. The vote was largely boycotted by unionist Catalans.
On 27 October, shortly after secessionist Catalan MPs voted to declare independence, the Spanish government of the then prime minister, Mariano Rajoy, won senate backing to sack Puigdemont and his cabinet and assume direct control of Catalonia.
Defence lawyers will seek to undermine the rebellion charges. They will point out that, under Spanish law, rebellion consists of “revolting violently and publicly”, and argue that none of those charged with the offence engaged in violence.
Speaking to the Guardian and a small group of other European reporters in Madrid on the eve of the trial, the Catalan president, Quim Torra, criticised the charges.
“The only violence we saw throughout September and October  was from the Spanish police,” he said. “They tried to stop citizens from going to vote peacefully on 1 October. But this has all been turned on its head. There was no violence, everyone saw there was no violence.”
The case will once again bring international attention to the enduring tensions between the Madrid government and the pro-independence regional government of Catalonia.
The president of the supreme court, Carlos Lesmes, has described the proceedings as “the most important trial that we’ve held since democracy [returned]”.
But on Monday night, Torra said: “As far as we’re concerned, Spanish justice has pretty much zero credibility.”
His sacked predecessor, meanwhile, called the trial a “stress test for Spanish democracy” and a test for the country’s judiciary.
Speaking in Berlin on Tuesday, Puigdemont described his former colleagues on trial as “honourable, innocent, democratic people,” and insisted there were “no criminal violations of the Spanish criminal law”.
The start of the trial led some Catalan pro-independence protesters to briefly block several roads before dawn, setting fire to tyres and holding up traffic. Protests were called in Barcelona for 7pm.