BARCELONA, Spain (Reuters) - Riot police briefly clashed with pro-independence Catalan protesters in Barcelona on Thursday while other activists burned tires and blocked highways across northeast Spain as part of a day of protests against the trial of 12 separatist leaders.

A total of 28 people, including twelve police officers, were wounded and four people were arrested, police said.

Near the Plaza Catalonia railway station in downtown Barcelona, police in riot gear used batons against a large group of mostly young protesters after a few of them hurled stones at the police.

One group of protesters disrupted underground city trains by sitting on tracks.

Protesters chanted “Independence, independence” and “Get out, occupation troops” in Catalan at the police.

Earlier in the day, protesters set fire to piles of tires to block some 11 highways. Police said roads had been reopened.

Members of local groups, working under the name Committees for the Defense of the Republic (CDRs), have staged periodic protests across Catalonia since separatist leaders were arrested after the Catalan parliament declared independence for the region in October 2017.

Twelve of the leaders went on trial in Madrid this month over the failed independence bid that triggered Spain’s biggest political crisis in decades.

Thursday’s protests included marches held by supporters of civil groups Omnium and the Catalan National Assembly (ANC) at midday, and a bigger march planned for the evening. The organizers also called a strike although it was not yet clear how far that was observed.

Former ANC leader Jordi Sanchez and the ex-head of Omnium, Jordi Cuixart, were in the dock on Thursday in a trial that is expected to last for at least three months.

The defendants face charges of rebellion, sedition and misuse of public funds, which they all deny.

The 2017 independence declaration, which followed a referendum conducted by Catalan authorities in defiance of a judicial ban, angered some in the region and many people in the rest of Spain.

The then conservative government in Madrid responded by imposing direct rule and then calling regional elections. The secessionists retained a slim majority in the Catalan assembly.