The people of Catalonia in north-eastern Spain are set to vote in an informal referendum on independence.
The Spanish judiciary has ruled the vote unconstitutional but Catalan leader Artur Mar warned against any attempt to disrupt it.
Spain's Prime Minister Mariano Rajoy said the vote would have no effect and urged the region to return to 'sanity'.
Voters will be asked whether they want a Catalan state and whether that state should be independent.
'Attack on democracy'
Catalonia is a wealthy a region of 7.5 million people and contributes more to the Spanish economy than it gets back through central government funds. Economic and cultural grievances have fuelled Catalan nationalism.
The BBC's Europe correspondent Chris Morris says many Catalans have always felt that they do not quite fit in a unitary state.
He says there is a long history of support for winning independence from Spain, or at least much greater autonomy within it.
This week, the Constitutional Court demanded the vote be suspended.
But Catalonia's government insisted it went ahead, organised by volunteers and with no official electoral roll.
Mr Mas warned the Spanish government against any attempt to halt the vote.
He said: 'I don't know what they will do, it does not depend on us, but if they have a minimum of common sense I think any action out of the ordinary would be a direct attack on democracy and a direct attack on fundamental rights.'
Mr Rajoy urged a return to sanity and for talks 'within the legal framework of the constitution'.
He said the vote would be 'neither a referendum nor a consultation nor anything of the sort'.
He added: 'What is certain is that it will not have any effect.'
Catalonia overview
Roughly triangular region in Spain's far north-east corner, separated by the Pyrenean mountains from southern France One of Spain's richest and most highly industrialised regions Many Catalans think of themselves as a separate nation from the rest of Spain This feeling is fed by memories of the Franco dictatorship, which attempted to suppress Catalan identity BBC profile: Catalonia
The Libres e Iguales (Free and Equal) group, which opposes the vote, held protests in dozens of cities.
One protest in Barcelona witnessed minor scuffles but no arrests.
Rallies in favour of the vote have also been held.
Murial Casal, the president of Omnium Cultural, which promotes Catalan culture, told a gathering on Friday: 'Call your parents, your friends and your neighbours and tell them that they have to vote.'
Catalonia adopted a charter in 2006 giving it 'nation' status but this was overruled by the Constitutional Court in 2010.
Entities 0 Name: Spain Count: 6 1 Name: Catalonia Count: 5 2 Name: Spanish Count: 3 3 Name: BBC Count: 2 4 Name: Constitutional Court Count: 2 5 Name: Europe Count: 1 6 Name: Mas Count: 1 7 Name: Catalonia The Libres Count: 1 8 Name: Mariano Rajoy Count: 1 9 Name: Artur Mar Count: 1 10 Name: Barcelona Count: 1 11 Name: Murial Casal Count: 1 12 Name: Pyrenean Count: 1 13 Name: Rajoy Count: 1 14 Name: Chris Morris Count: 1 Related 0 Url: http://ift.tt/10DVxHr Title: Catalonia: 'The more the state stops us, the more we will push for freedom' Description: The debate began on this quiet, leafy campus more than a month ago. In late September, when Spain's constitutional court agreed to hear the central government's challenge against Sunday's referendum on independence in Catalonia, resulting in the automatic suspension of campaigning, some at the University of Girona started raising questions about Catalan judge Encarna Roca.
