Police were guarding public buildings and closing off a park surrounding the regional Catalan parliament in Barcelona where a declaration of independence on Tuesday evening is likely to be met with a harsh response from Spanish central authorities.
Catalan president Carles Puigdemont has not revealed the precise message he will deliver in a plenary session at 6pm local time (1600GMT), but separatist politicians have said they expect a declaration based on the results of the disputed independence referendum on October 1.
The separatists have declared valid the pro-independence victory in the vote, which was followed by mass protests of Catalans angered by heavy-handed police tactics.
There have also been large-scale rallies by people committed to national unity.
How the declaration will be enforced and what the Spanish government's response will be are the key questions.
AP