Violence flared in Barcelona as angry Catalans protested against the detention of fugitive former leader Carles Puigdemont in Germany.
Police dressed in riot gear were striking demonstrators with batons as they tried to push back a large crowd advancing on the office of the Spanish government's representative in Catalonia.
Catalan police blocked the street and issued a call for people not to gather.
Thousands answered the call by a pro-independence grassroots group to protest in the city centre hours after Mr Puigdemont was detained by German police.
Breaking: Demonstrators clash with police in Barcelona after the arrest of former Catalan leader Carles Puigdemont in Germany. pic.twitter.com/YU18sYX7af— PM Breaking News (@PMBreakingNews) March 25, 2018
He fled from Spain in October following an illegal declaration of independence by the region.
Prosecutors in Germany said Mr Puigdemont will appear in court on Monday.
The Schleswig prosecutor's office said "the question of whether Mr Puigdemont has to be taken into extradition custody, will then have to be determined by the higher regional court in Schleswig".
It said the court will check, based on extradition documents by Spain, if the handover of Mr Puigdemont to Spanish authorities is "legally permissible".
Mr Puigdemont was detained on Sunday morning in Germany on an international warrant near the road leading from Denmark to Germany.
A Spanish Supreme Court judge charged 13 Catalan separatist politicians with rebellion on Friday for their attempts to make the region independent of Spain, dealing a heavy blow to the secessionist movement.
The judge ordered international arrest warrants for the six Catalan officials who are fugitives, including Mr Puigdemont.
Spain said its intelligence services and police worked with German police to orchestrate the arrest.
Mr Puigdemont was detained while travelling from Finland back to Belgium, where he has been living since October.
Elsa Artadi, a member of the Catalan parliament considered to be in Mr Puigdemont's inner circle, wrote on Twitter: "Spain does not guarantee a fair trial, only revenge and repression."