Five people were injured and seven arrested on Saturday as members of Spain's far-right Vox party demonstrating in Barcelona against Catalan independence clashed with separatist activists there, police said.
The violence came amid rising tension in the run-up to parliamentary elections next month and the continuing trial of 12 Catalan leaders for their part in the failed 2017 independence bid.
Police estimated that around 5,000 members of Vox demonstrated in the Catalan capital, in the words of their leader Santiago Abascal to "defend to the very end a united Spain".
"Spain will not resign itself to being destroyed," Abascal told the crowd.
Several groups of radical Catalan separatists, many of them masked, had organised counter-demonstrations around the Vox rally, which took place in the Plaza de Espana.
Carrying banners denouncing fascism about 300 of them set up barricades, burning rubbish bins, and throwing bottles and stones at officers, said police.
Vox made its electoral breakthrough when it won seats in the Andalusia regional assembly last December. It is expected to pick up more nationally in the April 28 parliamentary elections.
In the trial of the Catalan nationalists, it has taken the position of a "popular prosecutor", a role particular to the Spanish justice system, which allows it to be an accuser alongside state prosecutors.
Spain's socialist government has rejected suggestions that prosecution of the Catalan separatists has been unfair.