Catalan separatists are using 'Nazi' tactics to intimidate their opponents before an illegal referendum on independence, the Spanish Foreign Minister has claimed.

Axar.az reports citing Daily Mail that, Alfonso Dastis said separatists are encouraging their supporters to harass their opponents as tensions rise before the October 1 ballot to leave Spain.

'These people actually are taking some Nazi attitudes because they are putting up posters with the faces of mayors who are resisting their call to participate in this charade,' Mr. Dastis said.

He also said 'referendums are a weapon of choice of dictators' referring to the two the late Spanish leader General Francisco Franco conducted before the country returned to democracy after his death in 1975.

Mr. Dastis was speaking in an interview in New York, where he is attending a meeting of the General Assembly of the United Nations.

He spoke as the first of hundreds of Catalan mayors summoned to answer questions on why they have backed the banned referendum appeared before the state prosecutor on Tuesday amid cheers and chants from supporters.

Marc Solsona, mayor of the town of Mollerussa, one of nearly 750 mayors facing charges of civil disobedience, abuse of office and misuse of public funds said 'voting is not a crime,' as he left the state prosecutor's office in Barcelona.

Solsona smiled, kissed and gripped hands with dozens of clapping supporters gathered outside the state prosecutor's office as he entered to chants of 'You are not alone'.

'I'm just the mayor and I have to serve my people. I am committed to the people being able to vote on October 1 in accordance with the law passed by the Catalan parliament and what happens to me is not important,' he said.

Pensioner Angel Tena, 63, who had traveled to Barcelona to support the mayor said: 'We consider ourselves privileged to have a mayor who represents the townspeople above any other interests.'

Spain's Guardia Civil police have begun searching several headquarters in Barcelona of Catalonia's regional government, a Catalan government spokesman said.

Separately, police continued their search for ballot boxes, voting papers, and campaign leaflets on Wednesday, raiding the offices of Spain's biggest private delivery company Unipost in the Catalan city of Terrassa, Spanish media reported.

After a series of rulings from the Constitutional Court decreed the vote illegal, Prime Minister Mariano Rajoy is imposing financial curbs on the regional administration and has promised tougher sanctions still if officials refuse to back down.

Although polls show less than half of Catalonia's 5.5 million voters want self-rule, most in the wealthy northeastern region want the chance to vote on the issue.