Their televised trial, which is expected to last three months, is being seen as deepening the division between national government leaders in Madrid and pro-independents in Barcelona.
This trial, which is the biggest topic on social media in Spain, is bringing up a lot of memories from the referendum in October 2017, during which two million people voted for independence for Catalonia.
The Spanish government, and its security forces, faced allegations of heavy-handedness, both in their handling of the referendum, which Madrid said was illegal, and the way their supporters were treated by police.
Al Jazeera's Andrew Chappelle has more.
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