By Sam Edwards

BARCELONA (Reuters) - Tens of thousands of Catalan independence supporters clogged one of Barcelona's main avenues on Saturday to demand the release of separatist leaders held in prison for their roles in the region's banned independence drive.

Wearing yellow ribbons on their lapels to signify support, they filled the length of the Avenue Marina that runs from the beach to Barcelona's iconic Sagrada Familia church, while the jailed leaders' families made speeches.

Catalonia's two main grassroots independence groups called the march, under the slogan "Freedom for the political prisoners," after their leaders were remanded in custody on charges of sedition last month.

The protest is seen as a test of how the independence movement's support has fared since the Catalan government declared independence on Oct. 27, prompting Spanish Prime Minister Mariano Rajoy to fire its members, dissolve the regional parliament and call new elections for December.

"Look at all the people here," said 63-year-old Pep Morales, who was confident separatist parties would win the Dec. 21 election. "The independence movement is still going strong."

Many of the families, young people and pensioners there had travelled from across Catalonia to attend the march. They carried photos with the faces of those in prison and waved the red-and-yellow striped Catalan independence flag.

The Spanish High Court has jailed eight former government members, along with the leaders of the grassroots groups the Catalan National Assembly (ANC) and Omnium Cultural, while investigations into their roles continue.

The court last week issued arrest warrants on charges of rebellion and sedition for ex-regional president Carles Puigdemont, who flew to Brussels after being deposed, and four other former government members who went with him.

On Thursday, the Supreme Court released on bail the Catalan parliament's speaker Carme Forcadell and four other lawmakers, who enabled the declaration of independence by overseeing a parliamentary vote. Another lawmaker was released without bail.

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Forcadell was released after agreeing to renounce any political activity that went against the Spanish constitution, according to the court's ruling, in effect banning her from campaigning for independence in the December election.

Those terms threaten to undermine the independence movement just as cracks are starting to appear and tensions rise between the grassroots and their leaders.

The PDeCAT party of Puigdemont has failed to agree on a united ticket to contest the election with another secessionist party, denting the pro-independence camp's hopes of pressing ahead with its bid to split from Spain after the election.

On Saturday, the separatist Esquerra Republicana party said the ousted Catalan vice-president Oriol Junqueras, one of those in jail, would be its candidate in December and would campaign from behind bars.

Pepita Sole, a 61-year-old pensioner in the crowd on Saturday draped in a flag, said she understood the Oct. 27 declaration was symbolic but now wanted the real thing.

"They better understand that we're not faking," she said.









(Writing by Angus Berwick; Editing by Richard Balmforth)