Catalonia's surreal election campaign drew to a close Tuesday as separatists gathered to demand freedom for their detained leader, and axed regional president Carles Puigdemont prepared to hold a rally via videolink from exile in Belgium.

The regional vote on Thursday pits leaders of the wealthy northeastern region's separatist movement against candidates who want to stay part of a unified Spain.

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

The campaign has been closely watched from across the European Union, which is still reeling from Britain's decision to leave.

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.

"These elections will decide whether we return to normality, to the constitution, to reason," Spanish Prime Minister Mariano Rajoy told a rally in Barcelona Monday.

While 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.

"We're going to speak out so loudly that even Mr Puigdemont in Brussels will hear us," Arrimadas told a rally of supporters in Barcelona.

On Tuesday, she told La Sexta television that the election could ultimately be decided "by a few votes".

"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 added.

- Battle for separatist votes -

Spain recently dropped an international warrant for Puigdemont, but he will still be arrested should he return to Spain, where he is wanted on charges of rebellion, sedition and misuse of public funds.

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

His axed 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.

Allowed just 10 phone calls a week, Junqueras has led an unorthodox campaign, giving interviews to Catalan radio and sending articles, letters and even poems to supporters from behind bars.

But with Spain's government determined to stop the separatists in their tracks, the interior ministry launched a probe against Junqueras and another jailed secessionist over messages recorded from behind bars that were then played at campaign rallies.

On Tuesday, Junqueras's supporters gathered at the prison outside Madrid where he is being held to demand his release.

From there, they will travel to Junqueras's hometown near Barcelona, Sant Vicenc dels Horts, for a final party rally.

"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 gathering.

"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.

The separatist gathering was met with a counter-protest of two dozen far-right activists chanting "Puigdemont to jail!" and "separatists are terrorists".

- Possible deadlock -

The Catalan independence crisis came to a head on October 1, when the now deposed government went ahead and held a banned referendum.

But the vote was marred by a brutal police crackdown that sent shock waves around the world, triggering the worst political crisis in Spain in decades.

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

Their independence declaration ended in failure, with more than 3,000 companies relocating from the region, and no country recognising the new "republic".

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

If parties are not able to agree on a governing coalition, Catalonia could face fresh elections early next year, which would prolong political uncertainty in Spain, the fourth-largest economy in the 19-country eurozone.