Protesters in Madrid and Barcelona clashed with police for the second night in a row on Wednesday amid rallies in support of rap artist Pablo Hasel.
Hasel was arrested after barricading himself at a university with dozens of supporters to avoid prison and has portrayed his case as a fight for free speech.
A 24-hour standoff between police and Hasel ended early on Tuesday when anti-riot officers arrested him in northeastern Lleida University.
He was taken to prison to serve a 9-month sentence for insulting the monarchy and glorifying terrorism in a song about former King Juan Carlos I and in 64 tweets.
Protests have taken place across the country since Tuesday.
Emergency services said on Wednesday that 33 people were treated for injuries in the disturbances the previous day.
In Barcelona, thousands of protestors set bins on fire and threw rocks at the police.
Amnesty International says Hasel's case is the latest in a string of artists and social media personalities being put on trial for violating Spain's 2015 Public Security Law, which was enacted by a previous conservative-led government.
Many artists and celebrities including film director Pedro Almodóvar and Hollywood star Javier Bardem have shown their support and demanded a change to legislation.
Spain’s left-wing coalition government last week said it planned to change the country’s criminal code to eliminate prison terms for offences involving freedom of expression.