Catalan leader Carles Puigdemont said that he hoped the case would give Spain the opportunity "to impose a fair sentence: that is, acquittal".
Flanked by hundreds of police, pro and anti-separatist demonstrators gathered outside the Supreme Court, where the defendants face charges of rebellion, sedition and misuse of public funds, which they all reject.
Al Jazeera's David Chater reports from Madrid.
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