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.”