Catalan president Carles Puigdemont has not revealed the precise message he will deliver at 6pm local time (5pm UK time), but separatist politicians have said they expect a declaration based on the results of the disputed independence referendum on 1 October.
The vote had been declared illegal in Madrid before it even took place, and violent clashes erupted at polling stations.
Ahead of the announcement, Barcelona's mayor said the Catalan referendum is not enough for the region to declare independence from Spain.
Ada Colau said the results of the vote "can't be a guarantee to proclaim independence but are the opportunity to build dialogue and international mediation".
She appealed to Spain's prime minister and Catalonia's political leader to "decrease tension on both sides", adding that "now is the time to build bridges".
Ms Colau also urged Prime Minister Mariano Rajoy to pull out the extra police that have been drafted in to the northeast region as a "gesture of state responsibility".
As many in Spain await what would be a major escalation in the constitutional crisis, more firms have said they are pulling out of the region.
Highway management company Abertis and telecoms company Cellnex have joined a growing list of companies moving their headquarters elsewhere.
Both are part of Spain's Ibex 35 index of top listed companies.
Abertis and Cellnex said they were pulling out for as long as there is uncertainty about the region's future.
They join a slew of companies that are moving, including property group Inmobiliaria Colonial, Banco Sabadell, CaixaBank and energy firm Gas Natural.
Publishing house Grupo Planeta also warned it will move to Madrid if independence is declared.
Spain's deputy prime minister earlier warned of a tough response if Mr Puigdemont decides to announce a split from Spain.
Soraya Saenz de Santamaria told COPE radio that "if they declare independence, there will be decisions to restore the law and democracy".
He added: "I'm calling on the sensible people in the Catalan government... don't jump off the edge because you'll take the people with you."
Madrid has been criticised because of the heavy-handed approach by police.
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