Catalan president Carles Puigdemont has told the region's parliament he 'assumes the mandate' for an independent republic but asked for the effects of the declaration to be suspended in order to 'enter into dialogue'.
Donald Tusk has urged Catalonia's separatist leader not to take a decision that would make dialogue with Madrid impossible, just hours before a possible declaration of independence.
"We call on Puigdemont not to do anything irreversible, not to pursue a path of no return and not to make any unilateral independence declaration," government spokesman Iñigo Mendez de Vigo said.
"This regards a rebellion against the rule of law and the rule of law is the basis not only for coexistence in Spain but also for coexistence in Europe," Spanish economy minister Luis de Guindos said.
The Local asked international and Spanish constitutional law experts for their analysis of what the consequences of a Catalan unilateral declaration of independence would be.
Catalonia's drive to secede from Spain is rekindling dreams of independence in ethnic pockets across the Balkans, a potentially dangerous ambition in a region where nationalist violence claimed tens of thousands of lives in the 1990s.
Brussels has stuck to its line that an independent Catalonia would automatically be out and have to reapply to join, but some experts say pragmatism may yet trump dogma.
Catalonia, which may be poised to declare independence from Spain, is one of the powerhouses of the Spanish economy, buoyed by industry, research and tourism but burdened with heavy debt.
Catalonia, whose leader may declare independence on Tuesday, is one of the most strategic regions of Spain, the eurozone's fourth largest economy.
"We are athletes and politics is very far away."