Activists blocked roads and rail lines in Catalonia on Thursday while tens of thousands took to the streets during a region-wide strike against the trial of separatist leaders for their role in a failed 2017 secession attempt.
On Thursday evening 40,000 people rallied along one of Barcelona's major avenues, police said, while earlier around 1,000 demonstrators gathered in the central University Square, where the city's oldest university stands.
Four people were arrested with 37 people injured in clashes including six police. Called by a small pro-independence union, Intersindical CSC, but without the backing of larger unions, the one-day strike was not widely followed in the northeastern region.
Public transport in Barcelona was slower than usual but most shops remained open. Traffic authorities in Catalonia said activists 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.
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.
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"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 Catalan employers' association has denounced the work stoppage as politically motivated.
Separatist parties and associations are backing it. They began mobilising last week in solidarity with the Catalan separatist leaders on trial for pushing a secession referendum in October 2017 in defiance of a court ban, and a subsequent short-lived declaration of independence.
On Saturday, around 2,00,000 people including regional president Quim Torra protested in central Barcelona, police said, behind a banner that read "self-determination is not a crime."
This prompted unusually outspoken comments from King Felipe VI on Wednesday who said it was "unacceptable to appeal to a so-called democracy that is above the law."