The Spanish government has given the regional government in Catalonia 48 hours to abandon "illegal" referendum plans or lose budgetary powers.
Finance Minister Cristóbal Montoro said a mechanism had been approved for the state to take control of the autonomous region's finances.
Madrid is seeking to stop the Catalan government spending public money on its planned independence referendum.
The Catalans are defying a court order to suspend the 1 October vote.
Catalan President Carles Puigdemont launched his campaign for a "Yes" vote on Thursday night in the town of Tarragona, telling a rally at a former bullring: "Vote, and in so doing bring light to darkness that has lasted for too many years."
The crowd shouted back, "Independence", "We will vote" and "We're not afraid", AFP news agency reports.
Spanish Prime Minister Mariano Rajoy was taking the unionist cause directly to Barcelona on Friday, addressing a meeting of his Popular Party in the Catalan capital.
If the deadline is not met, the central government will take over the funding of most essential public services in the region, Mr Montoro said.