Twelve Catalan politicians and civic leaders are expected to be sentenced to "a maximum of 15 years in jail" by Spain's Supreme Court on Monday, amid an ongoing separatist crisis in the region. 

The jail terms could reignite tensions over a secession push that plunged the country into its worst political crisis in decades, two years after violent clashes between protesters and police.

A judicial source told Reuters news agency of the possible sentences on the day pro-Spanish unity demonstrations were held in the Catalan capital of Barcelona. 

The leaders are facing charges ranging from rebellion to sedition and the misuse of public funds for their failed bid in 2017 to create an independent Catalan republic. 

Charges are also linked to their involvement in organizing the 2017 independence referendum, when the region tried to break away from Spain.

The Spanish government declared the referendum illegal.

Former Catalan president Carles Puigdemont, Catalan deputy leader Oriol Junqueras, former Catalan regional parliament speaker Carme Forcadell, the Catalan National Assembly's (ANC) leader Jordi Sanchez and Omnium Cultural head Jordi Cuixart are amongst those on trial.

The verdict comes during the build-up to a second Spanish national election this year and could trigger potentially violent protests in the region.

On Saturday, thousands of Catalans calling for a "united Spain" marked the country's national day, marching through Barcelona waving both Spanish and Catalan flags.

One demonstrator told The Associated Press that "separatist politicians are trying to split Catalonia from the rest of Spain" and "truly deserve to be punished".