At least 460,000 people have signed up for a Barcelona rally to push for a "Catalan Republic", organisers said, though this would represent a marked fall from last year when a million people marched for separation
The demonstration will take place on a regional holiday commemorating Barcelona's defeat at the hands of troops loyal to Spain's King Philip V in 1714, and the region's subsequent loss of autonomy.
Since 2012, the holiday has been used by separatists to press for independence.
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This year's rally will serve also to gauge the group's strength after a referendum on full autonomy on October 1st last year, and the Catalan parliament's unilateral declaration of independence on October 27th, all came to naught.
"Separatists will hit the streets to show they are the most organised political force" in Catalonia, Oriol Bartomeus, a politics professor at the Autonomous University of Barcelona, told AFP.
Catalan president Quim Torra said the march will mark the start of a "mass mobilisation". Further protests are planned for the anniversary of last year's banned referendum, which was marred by clashes between police and voters.
"Our government has committed to implementing the republic," Torra said in a televised speech on Monday evening.
"This is a nation that feels and wants to be free."
Greater autonomy offer
Torra wants the central government in Madrid to allow a legally binding independence referendum for the region of 7.5 million people.
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Spain's conservative prime minister Mariano Rajoy imposed direct rule on partly autonomous Catalonia after last year's unauthorised plebiscite, and called early elections.
Separatist parties retained a majority, though slim, of seats in the regional assembly.
Rajoy's successor, socialist Pedro Sanchez, catapulted to power in June with the support of separatist parties and has taken a softer line on Catalonia.
Sanchez has offered the region a referendum on greater autonomy, but this was rejected by Torra insisting the October 1st "mandate" for full independence be respected.
Show of unity
There are growing divisions in separatist ranks -- between those wanting to provoke a clash with Madrid, and those seeking a more conciliatory approach.
"If a separatist is so naive or stupid to believe he can impose independence on the 50 percent of Catalans who are not (separatists), it's clear that they are mistaken," Joan Tarda, a lawmaker for separatist party ERC in the Spanish parliament, said last week.
The ERC has a softer approach than its ally in the regional government -- former Catalan president Carles Puigdemont's Together for Catalonia.
Puigdemont was sacked by Madrid after last year's independence declaration, and fled to Belgium.
Puigdemont remains in exile. Photo: AFP
Separatist leaders in jail or exile over their role in the separatist push, have urged supporters in a joint letter to turn out en masse for Tuesday's rally and not to give in to "provocations from those who seek to divide us".
Thirteen Catalan leaders have been charged with rebellion, which carries a jail term of 25 years.
"The most important message that we must send is to demand the release of political prisoners," referring to jailed separatists, Alex Budoy, a 59-year-old nurse, told AFP Monday evening at a "freedom march" on the eve of the holiday.
Polls show voters have not changed their views much over the past year.
A closely-watched survey by the Catalan government's Centro d'Estudis d'Opinio in July showed 46.7 percent of Catalans want an independent state, just ahead of 44.9 percent who said they were opposed.