Protesters in Barcelona set fire to cafe tables and chairs and tipped over rubbish containers on Thursday evening after thousands had demonstrated peacefully throughout most of the day.
It was the region's fourth day of protests that have led to the worst sustained street violence in Spain in decades after Catalan leaders were sentenced to lengthy jail terms for their roles in a 2017 bid for independence.
Young people draped in Catalan flags congregated peacefully, tossing balls and skipping rope. But later the mood turned ugly, with protesters starting fires on the Rambla de Catalunya street at the heart of the tourist district.
Earlier in the day, thousands of students took to the streets, some hurling eggs at police holding riot shields. Marches from around the region are due to converge on Barcelona on Friday and unions have called a general strike for the day.