The former head of Catalonia's regional police has stood trial in Madrid for his alleged role in the illegal bid for independence in 2017.
Josep Lluis Trapero, and three others, are charged with rebellion and sedition during the attempt by the Catalan regional government to break away from Spain.
The trial takes place as Prime Minister Pedro Sanchez promises to try and resolve tensions through political talks, instead of legal proceedings.
There are fears that secessionist feelings could reignite after several weeks of relative calm in the region.
The state prosecution has asked for Trapero to be sentenced to 11 years imprisonment, for allegedly conspiring with regional authorities during the failed secession bid.
Former Catalan regional President Carles Puigdemont fled to Belgium following the failed attempt for independence and became an MEP in May.
The European Parliament says they have begun to consider the possibility of lifting Puigdemont's parliamentary immunity, following the request by Spanish authorities.
Puigdemont's deputy, Oriol Junqueras, received a jail sentence for his role in 2017, but the EU's top court has ruled he was also covered by immunity as an MEP, leading to calls for his release.
The trial of Josep Lluis Trapero and other officials is expected to last two months.