Spain's prime minister on Monday accused Catalonia's separatist government of blackmail after a newspaper reported it was planning to declare independence unless Madrid allows the wealthy region to hold a referendum on secession.

"The threats and blackmail which have been put on the table are intolerable," Prime Minister Mariano Rajoy told a hastily convened press conference. What Catalan leaders "intend to achieve is the complete rupture of what Spain is today," he added.

Rajoy was reacting to a report in El Pais which said Catalonia's regional government was prepared to immediately declare independence for their northeastern region unless Spain's central government lets it hold a binding independence referendum.

The Catalan government has already drafted a transitional bill on the basic structure and functioning of an independent Catalan state, the newspaper said citing a copy of the blueprint.

The bill handles questions such as who would retain citizenship as well as provisions for taking over assets currently owned by the central government, according to the report.

The Catalan government denied the report, saying in a statement that its priority remains to reach an agreement with the central government to hold an independence referendum.

A Catalan government source told AFP that the document cited by El Pais was a "very preliminary draft written several months ago which is nothing like the text that exists today".

The report was published on the same day that the president of Catalonia, Carles Demounting, was due to give a highly anticipated speech at Madrid city hall to outline his government's plans for an independence referendum.

He has vowed to hold a referendum by September -- with or without the approval of the central government.

During his speech in Madrid Puigdemont pledged to go ahead with plans to hold the referendum and accused Rajoy of "not having done anything serious, sincere or real to tackle the Catalan problem". "We will never surrender in our intention of allowing Catalans to vote," he said in Madrid. "If there isn't an agreed proposal due to the lack of will of the Spanish government, the commitment of the Catalan government is democratically inviolable."

Some 200 protesters, many waving red and yellow Spanish flags, protests outside of city hall as Puigdemont arrived to give his address. They chanted "Catalonia is Spain" and insulted Catalan officials as they entered the building to hear Puigdemont speak.

Rajoy's conservative government argues that it could not permit such a vote since it would be unconstitutional -- a stance supported by the judiciary.

Parties that want Catalonia, which has its own distinct language and customs, to break away from Spain won a majority of seats in the regional parliament for the first time in 2015 local elections.

Demands for autonomy have been fueled by Spain's economic downturn, leading many to resent sending tax money to Madrid to prop up poorer regions.

Recent attempts by Madrid to interfere with Catalan education have further stoked passions as did a 2010 ruling by Spain's Constitutional Court that struck down part of a 2006 autonomy statute that recognized Catalonia as a "nation" within Spain.

Opinion polls show the vast majority of Catalans are in favor of holding a referendum but are evenly divided on independence.