The Spanish government is asking the country's constitutional court to suspend a bid by leaders in Catalonia to hold a referendum on independence.
Prime minister Mariano Rajoy made the announcement after an urgent meeting with members of his cabinet.
He said the vote, planned for October 1, is illegal and an attack on Spain's and Catalonia's institutional order.
In a televised appearance, the conservative leader said the vote does not have the democratic protections needed to be considered a referendum and promised it would not take place.
He also branded a parliamentary showdown on Wednesday to approve the referendum's legal framework a "political perversion" by the leaders of the Catalan government.
Earlier, Spain's leading prosecutor said criminal suits are being lodged to prosecute Catalan officials responsible for scheduling the vote on independence.
Chief state prosecutor Jose Manuel Maza said two different lawsuits are in the works, one that seeks to punish members of Catalonia's parliament who allowed the debate and vote on the legal framework of the referendum, and a separate one against the executive branch of the regional government, whose members officially called the referendum.
He said the officials could be charged with disobedience, abuse of power and embezzlement.
The state prosecutor's office has also instructed officials and police forces in Catalonia - the north-eastern region whose capital is Barcelona - to investigate and stop any actions taken towards the celebration of the referendum.