Catalan leader Carles Puigdemont is facing growing pressure to drop plans to break from Spain ahead of a key address to the regional parliament.
There is speculation he may announce a unilateral declaration of independence following a disputed referendum.
Catalan police have been posted outside parliament in Barcelona, sealing off the grounds to the public.
The mayor of Barcelona has urged Mr Puigdemont and Spanish PM Mariano Rajoy to “de-escalate” the crisis.
Mr Puigdemont’s address, which is due at 18:00 (16:00 GMT), comes after a vote was held on 1 October which Catalan officials say resulted in almost 90% of voters backing independence. 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.
What is Puigdemont likely to tell parliament?
Independence supporters have been sharing the Catalan hashtag #10ODeclaració (10 October Declaration) on Twitter.
Expectations are high that Mr Puigdemont will ask parliament to declare independence on the basis of the referendum law it passed last month.
Parliament, which is dominated by pro-independence parties, would then have 48 hours to vote.
Barcelona’s influential mayor, Ada Colau, has urged Mr Puigdemont not to declare independence. She 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.”
How is Madrid likely to react?
Mr Rajoy is due to appear in Spain’s parliament on Wednesday. He has already said any declaration of independence by Catalonia would “lead to nothing“.
Under Article 155 of the constitution, his Spanish government could suspend devolution.
The leader of Spain’s main opposition party, Pedro Sánchez of the Socialists, has said his party will back action by the government “in the face of any attempt to break social harmony”.
- What are the options now?
- Catalan Spaniards fear loss of identity
- Distressed police officer photo goes viral
What pressure is big business exerting?
A stream of companies have announced plans to move their head offices out of Catalonia in response to the crisis.
It is one of Spain’s wealthiest regions, accounting for a quarter of the country’s exports.
Publishing company Grupo Planeta said it would move from Barcelona to Madrid if there was a declaration of independence.
Other firms which have announced relocation plans include lenders Caixabank and Sabadell, infrastructure firm Abertis, property group Inmobiliaria Colonial and telecoms company Cellnex.
Do the separatists have support internationally?
The European Union has made clear that should Catalonia split from Spain, the region would cease to be part of the EU.
In France, government minister Nathalie Loiseau said that if there was a declaration of independence “it would not be recognised”.
A spokesman for German Chancellor Angela Merkel said she had “affirmed her backing for the unity of Spain” in a phone call with Mr Rajoy.
How did we reach this crisis?
Catalonia, a part of the Spanish state for centuries but with its own distinct language and culture, enjoys broad autonomy under the Spanish constitution.
However, a 2005 amendment redefining the region as a “nation”, boosting the status of the Catalan language and increasing local control over taxes and the judiciary was reversed by the Constitutional Court in 2010.
The economic crisis further fuelled discontent and pro-independence parties took power in the region in the 2015 elections.
Attempts to hold a legal Scottish-style referendum on independence have got nowhere with the Spanish government.