Spanish security operations to thwart a controversial secession referendum in the northeastern region of Catalonia were continuing on Tuesday as online operations rendered over 140 pro-independence web pages inaccessible.

Sources from the ongoing investigation into the referendum bid told EFE that among the sites seized by police was that of the Catalonian National Assembly (ANC), an organization that promotes the region's independence from Spain which later confirmed, however, that the web page was still accessible through its European Union domain name (.eu).

Other websites were replaced by a message from Spain's militarized police, the Guardia Civil, which read: "This domain has been seized and will be brought before a judge."

The European Commission spokesman, Margaritis Schinas, reiterated the institution's stance that the Spanish constitution should be upheld.

When asked for an opinion on the closure of pro-independence websites, however, European Commissioner for Digital Economy and Society Mariya Gabriel said it was not the job of the commission to become involved in such internal affairs.

Acting on judge's orders, the Guardia Civil has been tasked with confiscating all referendum-related material in Catalonia, including websites deemed to incite the separatist poll.

The referendum has already been suspended as unconstitutional by the highest rung of Spanish judiciary, the Constitutional Court.

Despite legal action orchestrated by Madrid, Spain's capital and seat of power, separatist officials in Catalonia, including regional president Carles Puigdemont, have pushed ahead with plans to hold the referendum on Oct. 1.

Madrid has begun buffering Guardia Civil numbers in Catalonia amid rising tensions in the affluent region.

A group of residents in Algeciras, a port city in the south of Spain, came out to show their support and waved Spanish flags for a unit of Guardia Civil officers leaving their station en route to Catalonia.

But the movement of Spanish police into Catalonia has triggered some tensions with the regional police, the Mossos d'Esquadra, whose highest ranking officer recently expressed his opposition to a national government plan to assume charge of coordinating his forces.