Voters are highly mobilised and a record turnout is expected, but with pro- and anti-independence candidates neck-and-neck in opinion polls neither side is likely to win a clear majority.

The election is being closely watched across a European Union still reeling from Britain's shock decision to leave and wary about any breakup of the eurozone's fourth largest economy.

It has inflamed passions not just in Catalonia but across Spain, whose government took the unprecedented step of stripping the region of its autonomy after its parliament declared independence on October 27.

But with their camp in disarray, secessionists would likely put their independence drive on hold should they win Thursday's vote.

"Even if a pro-independence government is formed it will be very cautious how it acts because it won't want to lose the restored authority the Catalan government has," Andrew Dowling, contemporary historian in Hispanic studies at Cardiff University told AFP.

"It won't want to see that suspended again," he added.

The deposed government's failed independence declaration saw more than 3,000 companies relocating from the region, and no country recognising the new "republic".

While opinion polls suggest a narrow lead for the leftist, pro-independence ERC, voters could ultimately hand victory to centrist party Ciudadanos, whose charismatic candidate Ines Arrimadas has campaigned on a fierce anti-nationalist ticket.

She is fighting to replace deposed president Carles Puigdemont, who is in self-imposed exile and wanted by the Spanish courts on charges of sedition, rebellion and misuse of public funds.

"We're going to speak out so loudly that even Mr Puigdemont in Brussels will hear us," Arrimadas told reporters in Barcelona, adding "a few votes" could decide the outcome.

"That's why I ask that no one stays home... to put an end to the nightmare of this independence drive and to start a new phase of reconciliation and common sense," she told La Sexta television.

Defying the odds, Puigdemont announced his candidacy for the leadership of Catalonia and is set to end his campaign with a rally that will be streamed via videolink across the region.

His former deputy, Oriol Junqueras, has been campaigning from jail, with his ERC party running on a separate ticket.

Now the separatists are not just fighting pro-unity parties for votes -- but each other.

"They (the government) want us subdued and demoralised, to surrender and give up," ERC parliamentary candidate Ernest Maragall said.

A sacked MP from Puigdemont's party, Antoni Castella, also attended the rally.

"We are here to condemn the absolute injustice of jailing innocent people... because of their non-violent ideas, which are representative of the views of the majority of Catalan citizens," Castella said.

A group of European parliamentarians tried, unsuccessfully, to visit Junqueras in jail.

"We came to uphold European fundamental rights under which all European lawmakers can go to prisons in all (EU member) countries to meet prisoners and to discuss the conditions under which they are being held," French MEP Jose Bove told reporters.

"We have just seen that in Spain that is not possible today," he said.

A Spanish government official said the MEPs had not obtained the necessary permits to visit Junqueras.

- Deadlock? -

Neither separatist nor pro-unity parties are predicted to win a decisive majority in the 135-seat parliament, which could lead to lengthy negotiations to form a regional government.

But separatists still appear to have a slight lead.

"Supporters of independence tend to vote more reliably, and the electoral system over-represents rural areas, where nationalist sentiment is strongest," said Ana Luis Andrade, Europe analyst at the Economist Intelligence Unit.

If parties cannot agree a governing coalition, Catalonia could face fresh elections next year, prolonging the political uncertainty.