Hundreds of thousands of Catalan independence supporters clogged central Barcelona 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.

Barcelona's police said that 750,000 people attended the rally. The protest is seen as a test of how the independence movement's support has fared since the Catalan government declared independence on 27 October, prompting Spanish Prime Minister Mariano Rajoy to fire its members, dissolve the regional parliament and call new elections for December.

Catalonia referendum protests – in pictures Show all 10 1 /10 Catalonia referendum protests – in pictures Catalonia referendum protests – in pictures Demonstrators block a Guardia Civil vehicle as they try to leave the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures Demonstrators react as they try to stop the car carrying Xavier Puig, a senior at the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office, after he was arrested by Guardia Civil officers in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures A demonstrator reacts as he tries with others to stop the car carrying Xavier Puig, a senior at the Department of External Affairs, Institutional Relations and Transparency of the Catalan Government office, after he was arrested by Guardia Civil officers in Barcelona AP/Emilio Morenatti Catalonia referendum protests – in pictures Spokeswoman of the Catalan pro-independence anticapitalist party "Candidatura d'Unitat Popular - CUP" (Popular Unity Candidacy), Ana Gabriel, talks to the media in Barcelona Josep Lago/AFP Catalonia referendum protests – in pictures Republican Left of Catalonia party's (ERC) Member of Parliament Joan Tarda (C) attends a demonstration outside the regional Economy Ministry in Catalonia during a police search for documents connected with the organisation of the Catalan independence referendum, in Barcelona EPA/Alejandro Garcia Catalonia referendum protests – in pictures A man holds pro-referendum poster next to a Spanish Civil Guard who stands in front of the Economy headquarters of Catalonia's regional government in Barcelona. The operation comes amid mounting tensions as Catalan leaders press ahead with preparations for an independence referendum on October 1 despite Madrid's ban and a court ruling deeming it illegal Josep Lago/AFP Catalonia referendum protests – in pictures People hold placards reading "Democracy" as they protest in front of the Economy headquarters of Catalonia's regional government in Barcelona AFP Catalonia referendum protests – in pictures A crowd of protesters gather outside the Catalan region's economy ministry after junior economy minister Josep Maria Jove was arrested by Spanish police during a raid on several government offices, in Barcelona Reuters/Albert Gea Catalonia referendum protests – in pictures People holding 'Esteladas' (Catalan pro-independence flags) attend a protest near the Economy headquarters of Catalonia's regional government Lluis Gene/AFP Catalonia referendum protests – in pictures People demonstrate on a Spanish Civil Guard Police car outside the Catalan Vice-President and Economy office as police officers holds a searching operation inside David Ramos/Getty Images

"Look at all the people here," said 63-year-old Pep Morales, who was confident separatist parties would win in the 21 December 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.

Ms 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 Mr 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 independence declaration was symbolic but now wanted the real thing.

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