Police said they carried out 10 raids and seized an abundance of material and substances they believe could be used to make explosives.
Spanish police arrested nine activists on Monday linked to Catalonia's pro-independence movement who may have been preparing violent actions, seizing material authorities believe could be used to make explosives.
The movement calling for Catalonia's separation from Spain has been a major challenge for the country for years, triggering its biggest political crisis in decades with a short-lived independence declaration in 2017.
However, it has largely been a peaceful movement characterized by mass protests, in contrast to a Basque separatist campaign that was for decades marked by a violent insurgency until ETA (Basque Homeland and Liberty) guerrillas ended their fight in 2018.
Police said the investigation and raids targeted local groups working under the name Committees for the Defence of the Republic (CDR), which have staged periodic protests across Catalonia since separatist leaders were arrested after the 2017 independence declaration.
In Sabadell, near Barcelona, the CDR called on its members to gather on Monday evening to protest the arrests.
"Self-determination is not a crime," the organisation said on Twitter.
Monday's raids were the result of an investigation launched more than a year ago by Spain's High Court, a spokesman for the Guardia Civil national police force in Catalonia said.
Acting interior minister Fernando Grande-Marlaska told public broadcaster RTVE that the details of the investigation remain secret.
"We can only say certain organisations have been charged with the potential of committing crimes of a violent character," he told RTVE.
Police said that more arrests could be made.
"So far, nine arrests have been made and 10 raids have been carried out in the province of Barcelona, with the purpose of locating and, where appropriate, seizing evidence of the degree of preparation of violent actions," police said in a statement.
La Vanguardia newspaper said the raids started at 5 am local time (0300 GMT) in various towns in Catalonia, including Sabadell and Parets del Valles.
Documents and computer hardware were seized, police said.
"In addition, we have found abundant material and substances which are considered precursors for the manufacture of explosives, susceptible (pending confirmation by specialists) of being used in the manufacture of explosive devices."
If it is confirmed that the materials seized on Monday were to be used in explosives, it would mark a first for CDR, a police spokesman said.
Last year, in a bid to mark the one-year anniversary of the region's independence referendum, members of the CDR occupied a train station in the Catalan city of Girona and blocked highways, forcing train cancellations and delays and snarling traffic.
They later attempted, but failed, to storm parliament.
Catalonia's independence is reaching a turning point with a verdict expected in the first half of October in the trial of 12 separatist leaders for their role in the 2017 independence referendum and declaration.
A poll published in July by a public Catalan institute showed support for an independent Catalonia at its lowest level in two years, with 48.3 percent of people against and 44 percent in favour.