Spanish Prime Minister Mariano Rajoy has demanded that the Catalan leader clarify whether he has declared independence, issuing a veiled threat that the central government could limit or rescind the province's autonomy if he has.
He said the Catalan government's response would be crucial in deciding "events over the coming days."
It is the first time that Mr Rajoy has openly said that Article 155 of the Spanish constitution will be the next step taken by the government if Catalan authorities don't backtrack. He said the government "wants to offer certainty to citizens" and that it is "necessary to return tranquility and calm."
Mr Rajoy issued the demand on Wednesday following a special Cabinet meeting to respond to an announcement from the head of the wealthy Catalonia region, Carles Puigdemont that he was proceeding with a declaration of independence but was suspending it for several weeks to facilitate negotiations.
In a highly anticipated speech on Tuesday night (local time), Mr Puigdemont said the landslide victory in a disputed October 1 referendum gave his government in the regional capital, Barcelona, the grounds to implement its long-held desire to break century-old ties with Spain.
But he proposed that the regional parliament suspend the effects of the declaration to commence a dialogue and help reduce tension, in what is Spain's worst political crisis in decades.
The central government in Madrid has given little indication it is willing to talk, saying it did not accept the declaration and did not consider the referendum or its results to be valid.
Article 155 of the Constitution allows the central government to take some or total control of any of its 17 regions if they don't comply with their legal obligations.
This would begin with a Cabinet meeting and a warning to the regional government to fall into line. Then, the Senate could be called to approve the measure.
Some 2.3 million Catalans - or 43 percent of the electorate in the northeastern region - voted in the referendum. Regional authorities say 90 percent were in favour and declared the results valid. Those who opposed the referendum had said they would boycott the vote.
Mr Rajoy's government had repeatedly refused to grant Catalonia permission to hold a referendum on the grounds that it was unconstitutional, since it would only poll a portion of Spain's 46 million residents.
Catalonia's separatist camp has grown in recent years, strengthened by Spain's recent economic crisis and by Madrid's rejection of attempts to increase self-rule in the region.
The political deadlock has plunged Spain into its deepest political crisis in more than four decades, since democratic rule was restored following the dictatorship of Gen. Francisco Franco.
Meanwhile self-proclaimed anarchists have burst into the Spanish embassy in Athens, throwing leaflets in favour of Catalonian independence.
The embassy staff evacuated the premises on Wednesday leaving only a few diplomats inside. There were no reports of damage or injuries.
"About 15 to 20 entered the embassy and threw leaflets. They did not break anything. The leaflet said 'solidarity is the weapon of the people'," an embassy official said.
The demonstrators unfurled a banner from the roof of the building reading "No Pasaran!" (They Shall Not Pass), a leftist slogan from the Spanish Civil War.
Greek police later detained 19 people, a police official said. Another police official said the group, known as Rouvikonas, had been demonstrating in favour of Catalonian independence.
"We are now back inside, the demonstrators have left," the embassy official said.
Small scale attacks against police, businesses, embassies and state buildings are frequent in Greece, which has a long history of political violence and is struggling to emerge from crisis despite three international bailouts.
Rouvikonas has stepped up its activities in recent months.