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, Independent reported.

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.

"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.