Vandalism, fires and throwing objects between police and protesters appeared on the streets of central Barcelona yesterday.
Protesters used garbage cans to build barricades against police and shop windows were smashed.
Several chanted "freedom for Pablo Hasél".
Demonstrations have also taken place in the cities of Vic, Lleida and Girona in the region of Catalonia.
Catalan regional police Mossos d'Esquadra wrote on Twitter that two people had been arrested in Barcelona, four in Vic and eight in Lleida.
Sentenced for tweets
Rapper Pablo Hasél barricaded himself at a university in Spanish Lleida in Catalonia on Monday after the deadline to surrender to the police to begin a nine-month prison sentence expired.
Police intervened and arrested him on Tuesday morning.
Pablo Hasél was convicted in 2018 for glorifying terrorism and insulting the royal family, mainly via Twitter, but also in lyrics.
Among other things, he has called the former king Juan Carlos a mafia boss and paid tribute to members of the terrorist group GRAPO.
Law change to wait
Several rappers have been convicted on similar grounds and the Spanish left-wing government has said it will review legislation to extend freedom of expression.
Among others, Amnesty has criticized Spain for using terrorism laws to convict musicians and artists.