Around 35,000 people demonstrated in Barcelona against a court ruling that more Spanish should be spoken in public schools.
The rally on Saturday was held under the slogan "Now and always, schooling in Catalan," the Europa Press news agency reported.
Speakers at the rally called on the regional government to disregard a contrary ruling by Spain's Supreme Court.
The judges had ordered that at least 25 per cent of classes be held in Spanish. The complaint was filed by the parents of a 5-year-old boy who attends a school in the municipality of Canet de Mar.
So far, most primary schools in the autonomous community in the north-east, where a significant part of the population wishes to become independent from Spain, have two 45-minute Spanish classes per week, with all other subjects taught in Catalan. However, many pupils speak Spanish outside of class.
Regional government leader Pere Aragones denounced the court's ruling as part of an "Spanish nationalism offensive."
Critics of the language model
Other speakers suggested that the advocates of more Spanish were really concerned with "less Catalan."
Critics of the Catalan language model, on the other hand, see the low proportion of Spanish in the classroom as part of Catalonia's efforts to break away from Spain.
Meanwhile, Catalonia's regional education minister announced inspections to ensure that only Catalan and no Spanish is spoken in class.
Almost all citizens of Catalonia know Spanish and some 80% Catalan. About one-third claim Catalan as their mother tongue.