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Protesters have blocked highways in Spain's northeastern region of Catalonia, as the trial of separatist leaders gets underway in the country's Supreme Court in Madrid.

Tuesday's protests, which have also led authorities to close off some main roads in the Catalan capital of Barcelona, were timed with the start of arguably Spain's most consequential trial in four decades of democracy.

Twelve defendants are being tried for their roles in pushing ahead with a unilateral independence declaration based on the results of a secession referendum that ignored a constitutional ban. Some face decades in prison if they are found guilty of rebellion.

The proceedings are being broadcast live on television.