Local media reported one of the group had also contacted Catalonia's regional president, Quim Torra, about alleged plans to occupy the regional parliament building in Barcelona, charges Torra's office has denied.
Catalans on trial in Spain
Protesters marched to the Sabadell town hall and demanded the separatists' immediate release, with leaders of the Libertad Detenidas 23S group claiming their incarceration was part of a political attempt to quash the peaceful independence movement.
Tensions are running high in Catalonia ahead of a Supreme Court verdict on the 12 leaders of the region's failed 2017 bid for independence. If they are found guilty, it will likely spark more protests.
Torra: Authorities manufacturing 'a violent narrative'
Catalonia's pro-independence parties this week demanded the withdrawal of national police from the region and amnesty for the 12 leaders, while endorsing civil disobedience campaigns.
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This prompted the federal government to warn against further pushes for secession. "This government is not willing to accept any attack ... on the constitution," warned government spokeswoman Isabel Celaa.
Torra said that by arresting the seven alleged terrorists, authorities were trying to "establish a violent narrative ahead of the verdict" on the 12 separatists on trial.
The alleged use of violence during the 2017 crisis is central to the prosecutors' case.