Brussels: The EU should do its duty as a repository of democratic values by telling Spain to allow a referendum on Catalan independence to take place, the region's foreign affairs chief said.

Authorities in Barcelona are determined to hold the vote as planned on Sunday, despite concerted opposition from the Spanish government, which has declared it unconstitutional and ordered regional police to take control of voting booths.

"We call on the EU institutions (...) to stand for the values and principles (of the EU treaty)," Raul Romeva told a news conference in Brussels.

"Civil rights are being violated ...and the quality of democracy in Spain is being eroded."

By failing to call for dialogue over the issue, the European Commission was being perceived as endorsing Madrid's "repressive action", he added, saying government officials, mayors and journalists were being subjected to harassment.

The Commission said it respected Spain's constitutional order.