Polls have opened in elections billed by Catalonia's leader as a referendum on its independence from Spain.
Artur Mas, the head of Catalonia's government, formed a separatist alliance ahead of the region's parliamentary elections and, should he win, plans to declare sovereignty within 18 months for the wealthy region's 7.6 million people.
"It is the most important election since Spain's return to democracy," the Madrid newspaper El Mundo declared.
Mas' liberal ruling party, the Democratic Convergence of Catalonia (CDC), is aligned with long-time rivals in the Republican Left of Catalonia (ERC) and the independence coalition Together for Yes.
The separatists' plan for independence is said to be the greatest challenge in Spain's recent history, with Prime Minister Mariano Rajoy saying his government would under no circumstances allow Catalonia's secession.
"No one will destroy the unity of Spain," he said.
According to surveys, Mas' alliance has reason to hope for a clear victory. Earlier polls have shown the separatists could emerge from the election as the decisively strongest power and might only narrowly miss an absolute majority.
EU politicians have suggested that an independent Catalonia would have to be automatically cut out from the European Union and the eurozone currency bloc.
Experts have also forecast heavy losses for the economies of Catalonia and Spain as a whole if the region secedes, with banks and large firms reportedly drawing up plans for a possible withdrawal from the region.
Mas wanted to hold a referendum for independence last November, but Madrid blocked the vote by filing a complaint to the Spanish Constitutional Court, which then declared such a vote illegal.
