The rally was organised by the Catalan National Assembly and Omnium Cultural public organisations, whose leaders were sentenced to nine years in prison each. The protests were supported by more than 100 organisations, associations and trade unions.
The protesters want the Spanish authorities to release the jailed Catalan politicians.
"We will go as far as the people want," Catalan President Quim Torra, who joined the rally, said.
The violent protests have been ongoing in Spain’s secessionist region since 14 October, when the Supreme Court sentenced several Catalan politicians to lengthy prison terms over their involvement in the semi-autonomous region's 2017 unsanctioned independence referendum. Hundreds of protesters have since been injured and detained.
At the same time, on Saturday, the right-wing Vox party organised a rally "in defence of the unity of Spain" in central Madrid that gathered 20,000 people. Vox leader Santiago Abascal called on the Spanish government to ban the Catalan pro-independence parties, suspend Catalonia’s autonomy and detain Torra. On Sunday, Barcelona will also witness a rally in support of Catalonia’s unity with Spain.