Catalan leader Carles Puigdemont is due to announce his plans for the future of the province amid growing pressure to drop plans to break away from Spain.
There is speculation he may announce a unilateral declaration of independence following a disputed referendum.
His address to the regional parliament is scheduled to take place at 18:00 (16:00 GMT).
Catalan police have been posted outside the parliament in Barcelona, sealing off the grounds to the public.
The mayor of Barcelona has urged Mr Puigdemont and Spanish Prime Minister Mariano Rajoy to "de-escalate" the crisis.
The Madrid government called on the Catalan leader "not to do anything irreversible, not to pursue a path of no return and not to make any unilateral independence declaration".
And the president of the EU's European Council has appealed to him to respect Spain's constitutional order.
A vote was held on 1 October which Catalan officials say resulted in almost 90% of voters backing independence for the north-eastern region. Turnout was put at 43%.
The vote was deemed illegal by Madrid and suspended by Spain's Constitutional Court. "No" voters largely boycotted the ballot and there were several reports of irregularities. National police were involved in violent scenes as they manhandled voters.