Traffic authorities in the northeastern region said they had cut off around 20 roads, including the A7 highway that links Spain to France, the A2 between Madrid and Barcelona and the main entry points to the Mediterranean city.   

READ MORE: Catalan separarists' trial: What you need to know

Police cleared several of the road blocks and detained two protesters, a spokesman said.

Activists also briefly blocked train traffic by occupying the tracks at a station in Barcelona and in other parts of the region, according to the company that manages Spain's rail network.   

At midday, hundreds protested in the central University Square, where the city's oldest university stands, and another demonstration is planned for the evening.

“We're here in solidarity with those who are victims of a trial that just doesn't hold water,” said Jaume Sole, a 45-year-old engineer.

The one-day strike has been called by Intersindical CSC, a small pro-independence union, to protest against the trial of 12 separatist leaders that opened last week at Madrid's Supreme Court.

The union has the support of separatist parties and associations, but not of the other bigger unions.

The Catalan employers' association, meanwhile, has denounced the work stoppage as politically-motivated.   

The region's separatist government has expressed solidarity with the strike and cancelled all official events planned for Thursday.   

The trial of 12 Catalan separatist leaders and activists, nine of whom are accused of rebellion, started on February 12th under intense domestic and foreign scrutiny.

They are being tried for pushing an independence referendum in October 2017 in defiance of a court ban, and for a subsequent short-lived declaration of independence on October 27th.

Catalonia's former president Carles Puigdemont, who fled for Belgium soon after, is not among the defendants as Spain doesn't judge people in absentia for major offences.

READ ALSO: Spain's King Felipe speaks out during Catalan separatists' trial