0255 GMT July 19, 2018
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 color chosen to show solidarity with the jailed leaders, AFP reported.
The march was called by a platform set up in March to "defend Catalan institutions" and "the rights and fundamental freedoms" of its citizens.
The protest was backed by the Catalan branches of Spain's two largest trade unions, the CCOO and the UGT, sparking unease among union members who oppose independence for the wealthy northeastern region.
The demonstration came ten days after a German court dismissed an extradition request for Catalonia's ousted separatist president Carles Puigdemont on grounds of rebellion and released him on bail.
Spanish prosecutors last week handed over new information to Germany they hope will prove the use of violence which would justify the rebellion charge against Puigdemont and their extradition request.
Puigdemont is also accused of misuse of public funds for staging an independence referendum in Catalonia on October 1 despite it having been ruled unconstitutional by the courts.
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.
Prosecutors say the two men played central roles in orchestrating pro-independence protests in September in Barcelona which trapped national police inside a government building and destroyed their vehicles.
They are also accused of mobilizing thousands of pro-independence supporters to prevent police from stopping the October 1 independence referendum from going ahead.
Six other Catalan separatist leaders are in jail in Spain for rebellion, along with Junqueras and the leaders of the grassroots separatist groups.
Catalonia has been in political limbo since Spain's conservative central government imposed direct rule on the region after it unilaterally declared independence in October.
Fresh regional elections will be triggered if a new leader is not elected by May 22.