Thousands of people backing Catalonia's bid to secede from Spain are staging a rally in Barcelona to demand the release of jailed separatist leaders.

The rally's grassroots organisers are calling for 10 prominent members of the secessionist movement in the north-eastern Spanish region to be freed from prison.

Eight former members of Catalonia's dissolved Cabinet and two activists are in jail while investigations continue into their alleged roles in promoting an illegal declaration of independence last month.

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Polls show that Catalonia's 7.5 million residents are roughly split over remaining a part of Spain.

Spain's constitution says the nation is "indivisible" and that questions of national sovereignty pertain to the national parliament in Madrid.