The former police chief of Catalonia goes on trial Monday over his role in the region's failed 2017 independence bid in a move that risks fanning tensions with Catalan separatists.
Josep Lluis Trapero, former head of the Mossos d'Esquadra regional police force, will face the court on charges of rebellion with prosecutors seeking an 11-year jail sentence.
During the October 1 independence referendum, which was banned by Madrid, prosecutors say the Mossos demonstrated "total passivity" under Trapero's watch while Spain's national police led a violent crackdown, using batons and firing rubber bullets in a bid to stop voters.
The trial of the career police officer comes three months after Spain's Supreme Court sentenced nine Catalan separatist leaders to long prison terms, prompting weeks of protests which sometimes turned violent across the wealthy northeastern region.
The trial opens at 10:00 am (0900 GMT) at a court in San Fernando de Henares on the outskirts of Madrid.
Another senior police officer and a former official with the Catalan interior ministry will also be in the dock facing charges of rebellion while a fourth person, a Mossos officer, is facing the lesser charge of sedition.
The trial, which is expected to run until March 19, will focus on the Mossos' role in enforcing a court ban on the referendum, which was followed several weeks later by a short-lived declaration of independence.
Several of the jailed Catalan leaders have been called to testify.
In a precedent that may help Trapero, the Supreme Court in October dismissed the charge of rebellion against the nine separatist leaders, finding them guilty of sedition, which carries a shorter prison term.
By definition, rebellion is "rising up in a violent and public manner" to, among other things, "declare independence for part of the (Spanish) territory".
Sedition, however, is "rising up publicly and in turbulent fashion" to "prevent by force or in an illegal way" the law from being applied, or the application of an administrative or legal decision.
Trapero appeared at the separatists' trial as a witness, defending his performance as police chief and saying the Mossos "had no intention of facilitating the referendum".
He also said his officers would have been ready to arrest then Catalan president Carles Puigdemont had they been asked. Puigdemont fled Spain to avoid prosecution.
Shortly after the Catalan parliament issued a short-lived declaration of independence, Madrid sacked Trapero, dismissed the regional government and temporarily took over Catalonia's regional affairs.
His dismissal came just two months after he was praised for capturing those behind a summer van attack that killed 14 people in Barcelona and was claimed by Islamic State extremists.
The trial was to open just a week after Socialist Prime Minister Pedro Sanchez's new leftwing coalition government took office in a move that was facilitated by the support of a Catalan separatist party.
In exchange, Sanchez promised to open talks with Catalonia's separatist regional government over a solution to the "political conflict".
He also promised to stop systematically challenging separatist initiatives as had been the case with previous administrations in Madrid.
In another development, Sanchez nominated former justice minister Dolores Delgado as prosecutor-general last week in a move that drew criticism for blurring the lines between the executive and the judiciary.
The role could influence future legal cases involving Catalan separatist, as well as how quickly those leaders who were jailed in October could obtain parole.