Protests broke out across the Spanish region of Catalonia on Sunday after former leader Carles Puigdemont was taken into custody in Germany.
At least 89 people were injured in clashes with police and four arrests were made.
Mr Puigdemont, who is wanted in Spain for sedition and rebellion, was detained by German police acting on a European arrest warrant.
He will appear before a German judge later on Monday.
Mr Puigdemont was detained while crossing from Denmark on his way to Belgium, where he has been living in self-imposed exile since Catalonia’s parliament unilaterally declared independence from Spain in October.
A European warrant for his arrest was reissued on Friday.
In central Barcelona, protesters chanted “Freedom for the political prisoners” and “This Europe is shameful!” as they headed to the offices of the European Commission and the German consulate.
Spanish news agency Efe estimated crowds of 55,000 in the centre of the city.
Smaller demonstrations were held in Girona, where Mr Puigdemont once served as mayor, Tarragona and Lleida. Some protesters also formed road blocks in various locations.
Tensions in Catalonia are very high and its separatist leaders abandoned plans to name a new president after the arrest on Friday of the latest candidate, Jordi Turull, sparked protests in Barcelona.