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.
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.
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.
What security measures are being taken?
The Catalan police force, the Mossos d'Esquadra, closed the Ciutadella Park, where parliament is located, early in the day, positioning vans near the entrances and alongside parliament.
The authorities said the measures were "for safety reasons".
The Catalan National Assembly (ANC), a non-party grassroots movement, earlier urged supporters in a tweet to come to the parliament district to "defend" the vote for independence.