Spanish prime minister Mariano Rajoy has rejected calls by Catalan officials for negotiations on a referendum on the region's independence, saying the law does not allow it.
Officials in Catalonia and the north-eastern region's capital, Barcelona, asked in a letter released on Friday for fresh dialogue on holding the vote with the government's permission.
"The prime minister can't make something illegal into something legal," said Inigo Mendez de Vigo, the minister of culture.
The central government says constitutional reform through a strong majority in the national parliament is the only avenue for a legal referendum.
Madrid has rejected the unilateral vote planned for October 1, but separatist politicians launched the Yes campaign on Thursday as they press ahead with the vote despite a ban by the country's courts and a criminal investigation into three out of four Catalan mayors actively supporting it.
In the letter, Catalan president Carles Puigdemont and Barcelona mayor Ada Colau also say Spain has launched "an offensive of repression without precedent".
The letter says a copy is being sent to King Felipe VI.
Meanwhile, Spain's finance minister has said the central government has decided to take over payments of essential services in Catalonia.
Cristobal Montoro said the government is also giving officials in Catalonia 48 hours to comply with a new system to scrutinise public payments to ensure that no public funds are being used on the illegal vote.
Mr Montoro's ministry ordered regional authorities in July to send weekly spending reports instead of monthly reports, but on Thursday, Catalan vice president Oriol Junqueras, who is in charge of economic affairs, said he would stop providing them because the scrutiny was politically motivated.
The extraordinary measures were justified, Mr Montoro said, for budgetary stability in Catalonia and to defend Spain's legal order.
AP