The announcement came almost immediately after the Catalan leader, Carles Puigdemont, facing a second deadline to clarify Catalonia's intentions since it held an Oct. 1 referendum on independence, warned that regional lawmakers were prepared to break from Spain.

The government in Madrid, in turn, announced that it would convene an emergency cabinet meeting on Saturday "to defend the general interest of Spaniards, among them the citizens of Catalonia."

The rapid succession of events moved what was already one of the gravest crises in Spain's relatively young democracy to a far more serious and unpredictable stage, with the prospect that Madrid could take over the running of Catalonia. At the most extreme, the Spanish government could arrest Mr. Puigdemont and charge him with sedition, as it has done with two other separatist leaders.