Demonstrators shone their mobile phones in unison at sunset to show solidarity with 10 prominent members of the independence movement.
Eight former members of the region’s dissolved government and two activists were jailed while Madrid investigates their alleged roles in organising an illegal declaration of independence last month in violation of Spain’s constitution.
Police estimated that 750,000 people attended the rally.
Many of the protesters carried pro-independence estelada flags.
Many also held signs saying in Catalan “Freedom Political Prisoners” and wore yellow ribbons as a symbol of their demands.
Family members of the jailed separatists read messages from their loved ones to the crowd at the conclusion of the march.
Grassroots group National Catalan Assembly organised more than 500 buses to bring people from towns and villages across Catalonia to its main city of Barcelona.
Meanwhile, small sister demonstrations were staged in both Glasgow and Edinburgh this afternoon.