Two prominent Catalan politicians are testifying before a Supreme Court judge for their roles in holding a banned independence referendum and making an illegal declaration of independence based on its results.
Judicial police have identified left-republican ERC party's secretary-general, Marta Rovira, and conservative PDeCAT's president, Marta Pascal, as key players in the secession bid in October.
The Spanish government responded by disbanding the regional government and calling a new Catalan election. Separatist parties have since been entangled in endless negotiations on how to form a new government.
Judge Pablo Llarena could decide after Monday's hearing whether to send the two politicians to jail while the investigation continues.
Other separatist leaders have been jailed and five former Catalan Cabinet members, including ex-president Carles Puigdemont, have fled to Belgium.