Spanish police arrested nine pro-independence activists in Catalonia on Monday morning on charges of terrorism, rebellion, and possession of explosives.

According to the public prosecutor, the detainees belonged to a "Catalan secessionist terrorist group" and were in "advanced" preparations to carry out violent actions in the coming weeks.

The Guardia Civil police force said they had carried out ten raids in the operation, and seized large quantities of material and substances they believed could be used to make explosives.

The activists are members of the Committees in Defense of the Republic (CDR), a network of local protest groups created two years ago in the run-up to the independence referendum as Spanish police were trying to dismantle preparations for the unauthorised vote.

The CDR called the arrests "arbitrary" and accused Spanish authorities of “persecuting" members of the Catalan independence movement.

Dozens of pro-independence supporters took to the streets to protest against the police operation in the Catalan town of Sabadell, where some of the raids took place.

In all, more than 500 police officers from the Guardia Civil were deployed in Catalonia to carry out the arrests and raids, mostly in the Vallès area near Barcelona. Apart from Sabadell, police were also sent to the towns of Mollet del Vallès, Cerdanyola del Vallès, and Sant Vicenç de Torelló.