The former speaker of the Catalan parliament is appearing at Spain’s Supreme Court over her role in the region’s push for independence.
Judges may decide to remand Carme Forcadell and five former lawmakers in custody on charges of sedition, rebellion and misuse of public funds.
Eight sacked members of the Catalan government are already being held in detention in a parallel investigation.
Spain’s Constitutional Court ruled the Catalan bid for independence illegal.
The country has been gripped by a crisis since a referendum on independence from Spain was held in Catalonia on 1 October in defiance of the central government in Madrid.
After the Catalan parliament voted to declare independence on 27 October, the Spanish government took direct control of the region and sacked its leaders.
As Ms Forcadell entered court on Thursday she was jeered by supporters of Spanish unity.
She and her five colleagues were part of the body that organised the affairs of the Catalan parliament.
They are accused of following a “concerted strategy to declare independence” that led to Catalonia announcing it was breaking away.
Ms Forcadell has always maintained that holding discussions and votes in a democratically-elected parliament is not a crime.
The Supreme Court in Madrid is dealing with the case because the six had parliamentary immunity at the time of the alleged offences. A previous hearing earlier this month was postponed until Thursday.
A decision to jail them pending further investigation would probably spark further protests by supporters of Catalan independence.
They have already staged blockades of roads and rail stations, angered at the detention of the eight other politicians and two activists they see as political prisoners.
A Spanish judge is also seeking the extradition from Belgium of five others – including the former Catalan leader, Carles Puigdemont.
A Belgian judge has yet to decide whether to execute an EU arrest warrant.
Mr Puigdemont has been freed on bail and is scheduled to appear in court on 17 November.