Catalan President Carles Puigdemont is facing growing pressure to drop plans to break from Spain ahead of a key address to the regional parliament.
There has been speculation that he could announce a unilateral declaration of independence following a disputed referendum.
France and Germany have both expressed support for a united Spain.
The mayor of Barcelona urged Mr Puigdemont and Spanish PM Mariano Rajoy to “de-escalate” the crisis.
Ahead of Mr Puigdemont’s speech to the Catalan parliament on Tuesday evening, Mayor Ada Colau urged him not to declare independence, and also called on Mr Rajoy to rule out direct control from Madrid.
“I ask them not to take any decision that might blow up the possibility of any space for dialogue and mediation,” said Ms Colau.
“That is the most courageous act they could do now.”
The leader of Spain’s Socialist opposition party, Pedro Sanchez – who is also opposed to independence – weighed into the row.
He told reporters in Barcelona that he would back action from the Madrid government “in the face of any attempt to break social harmony” if independence was declared unilaterally.
The Spanish government has threatened to suspend the autonomy of the north-eastern region.
Economic pressure is also increasing on Catalan separatist leaders with more major companies moving head offices away from the region.
Infrastructure firm Abertis, property group Inmobiliaria Colonial and telecoms company Cellnex all announced plans to relocate to Madrid.
Analysis: Will Puigdemont make good his threat?
Tom Burridge, BBC News, Barcelona
Never before has the world watched Catalonia’s regional parliament so closely. This evening, with probably large numbers of police and protesters outside, the leader of the devolved government is due to address Catalan MPs.
Carles Puigdemont plans to present his government’s results of the disputed referendum, which Madrid declared illegal.
Since then, Mr Puigdemont has said on several occasions that he will declare independence, despite fierce opposition across Spain and criticism from European governments.
Now, will he make good his threat? Or will he announce a more nuanced strategy, hoping still for a proper, recognised referendum one day? Under pressure to act, the Spanish government has made stark warnings, too. If the order comes, thousands of Spanish national police, here in Barcelona, could intervene.
Publishing company Grupo Planeta said it would also move from Barcelona to Madrid if there was a declaration of independence.
Major lenders Caixabank and Sabadell have already announced they are moving offices out of the region.
Catalonia is one of Spain’s wealthiest regions, accounting for a quarter of the country’s exports.