Catalan activists protesting the trial of 12 separatist political leaders burned tires, clashed with police and blocked major roads across Spain’s northeastern region on Thursday.

At least 11 highways in Catalonia were affected by the strike organized by small unions of pro-independence workers and students. Police said three people were arrested when they met resistance trying to clear groups who had stopped traffic.

Authorities said one protester was arrested for allegedly hitting a police officer in downtown Barcelona, while a second person was arrested on a highway north of the city where protesters burned tires and threw rocks at a line of riot police.

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Twelve officers also received minor injuries in the clashes, police said.

Members of local groups, working under the name Committees for the Defense of the Republic (CDRs), have staged similar demonstrations across the region since the arrests of the Catalan police leaders who illegally declared secession in the regional parliament.

On paper, the groups are demanding improved social policies including a 35-hour work week and a higher minimum wage, but they have carried pro-secession flags and chanted slogans for the release of the 12 political leaders on trial in the Madrid-based Supreme Court.

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The main unions in Catalonia did not back the strike, which appeared to have a limited impact on businesses.

The trial into the roles played by the 12 separatists in Catalonia's failed 2017 secession attempt is in its second week. On Thursday, former Catalan government member Santi Vila, ANC's ex-president Jordi Sanchez, and fellow activist Jordi Cuixart were scheduled to testify at the court.

Defendants face charges of rebellion, sedition, and misuse of public funds, which they all deny. The trial expected to last at least three months.

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Election results and polls indicate that Catalonia's 7.5 million residents are divided down the middle over the secession issue.

The Associated Press contributed to this report.