Carme Forcadell was held by Spain's Supreme Court in Madrid pending bail of 150,000 euros, a court spokesman said.
Forcadell and five other deputies appeared in front of Spain's highest tribunal on charges of sedition, rebellion and misuse of public funds -- which carry a maximum jail sentence of 30 years -- for aiding the Catalan parliament's secession bid.
They are suspected of having followed a "concerted strategy to declare independence", before the official declaration on October 27, deepening Spain's most serious political crisis in decades.
Supreme Court judge Pablo Llarena ordered one Catalan lawmaker freed, but the four others apart from Forcadell must pay 25,000 euros within a week to avoid jail.
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The Catalan crisis has prompted hundreds of businesses to re-register outside the wealthy northeastern region and caused disquiet in a European Union still dealing with Britain's shock decision to leave the bloc.
Yesterday, a general strike called in Catalonia by a pro-independence union triggered widespread travel chaos, cutting Spain's main highway link to France and the rest of Europe and disrupting trains from Barcelona to Paris, Marseille and Lyon.