Prosecutors say the two men played central roles in orchestrating pro-independence protests in September in Barcelona during which national police were trapped inside a government building for several hours and their vehicles were destroyed.
Many chanted "Freedom for the political prisoners" as they massed on the Parallel Avenue, one of the city's main streets, wearing yellow scarves, sweaters or jackets - the colour chosen to show solidarity with the jailed leaders. The local authorities estimated that 315,000 people assembled in the Plaça d'Espanya for the midday march, with more than 950 buses bringing in supporters from across Catalonia.
The regional chapters of Spain's two leading labour unions, along with other civil society groups, supported the protest despite the complaints from some members who don't want secession for Catalonia.
The Catalan sections of CC.OO. and UGT defended their attendance to the mobilization, after considering that the serious dispute in the rich autonomous community of 7.5 million inhabitants must be resolved through dialogue and politics and not in the courts.
Alex de Ferrer, a 50-year-old IT specialist, said he'planned to take part in the protest because jailing separatist leaders "only serves to manufacture separatists".
The wealthy northeastern region remains in a state of political chaos.
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The independence movement is "a bit decapitated" following the arrests of its leaders but "this is temporary", added de Ferrer, who supports leftist separatist party ERC, whose leader Oriol Junqueras is in jail.
Lawmakers defied court orders and held that a referendum on autonomy. Their subsequent declaration of independence for the region led to a crackdown by Spanish authorities acting to defend the Spanish Constitution, which declares the nation "indivisible".
However, in those elections, held on December 21, the three pro sovereignty parties revalidated their majority in the Catalan chamber although the legislature has not yet begun after four unsuccessful attempts to invest in a new President.
Since October 16 the leaders of Catalonia's two largest pro-independence groups - Jordi Sanchez of the ANC and Jordi Cuixart of Omnium Jordi Cuixart - have been in jail while they await their trial for rebellion.
Fresh regional elections will be triggered if a new leader is not elected by May 22.
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