Security was tight in the Spanish region of Catalonia on Wednesday, as police braced for further protests after riots broke out on Tuesday over the arrest of a rapper on charges of glorifying terrorism and insulting royalty in his songs.

Pablo Hasel, known for his radical leftist views, had missed a deadline last Friday to surrender to authorities to serve a nine-month jail term handed down in 2018 - a sentence that caused an uproar in Spain and led the government to announce it would make free speech laws less restrictive.

Police on Tuesday stormed a university building where Hasel had barricaded himself and arrested him, triggering rallies and riots in Barcelona and other Catalan cities.

Some protesters set fire to trash containers, looted shops and clashed with police who used batons and foam projectiles on various occasions.

Police arrested 18 people, while 55 were injured, including 25 officers, officials said.

Protest organiser used social media to call for further rallies on Wednesday night to demand Hasel's release across Spain, including in Madrid.

A source at Catalonia's interior department said police had “strengthened sensitive areas” to avoid more riots, but declined to give further details.

During a news briefing, regional security chief Miquel Samper appealed to protesters to congregate peacefully.

A police source said there was no plan to bolster security nationwide for the protests, however.

(With input from agencies)