The first of hundreds of Catalan mayors summoned to answer questions on why they have backed a banned referendum on independence from Spain have appeared before the state prosecutor amid cheers and chants from supporters.

The first three mayors to declare exercised their right to remain silent, the Association of Municipalities for Independence said.

Years of separatist feeling in the industrial northeastern region will come to a head in less than two weeks as the fiercely pro-independence regional government holds a referendum on splitting from Spain on 1 October.

The Spanish government has declared the referendum illegal and the Constitutional Court has suspended the vote that was approved by the regional government earlier this month.

So far, 745 of 948 municipal leaders have said they will provide venues for the referendum.

"Voting is not a crime," said 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, as he left the state prosecutor's office in Barcelona.

"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.

Mr 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'.

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

Neither the police nor the Interior Ministry could confirm the raid, but footage showed dozens of people gathered outside the company's offices chanting 'Out with the occupying forces,' handing out voting papers and laying carnations on police cars.

Unipost confirmed the raid without giving further details.

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.