Carles Puigdemont's detention threatens to worsen the Catalan crisis that flared last year when the region made a symbolic declaration of independence,
Clashes have broken out in Barcelona after the arrest of former Catalan leader Carles Puigdemont.
He was taken into custody by German police on Sunday after Spain issued an arrest warrant.
As news emerged of his detention, thousands took to the streets to denounce the move.
One protester said: "It's a mix of rage, sadness, fear, disappointment and defenselessness in face of everything that happens in Catalonia - and the fact that rest of the liberal opinion in Spain is staying silent."
Another said: "I think most of the people gathered here are leaning towards the idea of a general strike, an indefinite general strike, because to face repression one can only take to the streets, paralyse the country and take down its infrastructures and media."
Waving Catalan pro-independence flags - and calling for political prisoners to be released, demonstrators marched on the Spanish government's headquarters.
Gabriel Colomé, Professor of Political Science at the Universitat Autònoma de Barcelona, said: "The conflict could worsen If the leadership on the streets in taken by the radical CDR party, the Committees in Defense of the Republic. We could have a problem because the situation could become like Nortthern Ireland. They don't mind if there would be no economic recovery, or no more integration into europe - they are anti-Europe and anti-system."
Our correspondent Cristina Giner says: Puigdemont's expected appearance in a German court has upped the ante with some calling for a sustained general strike.