Protests over the imprisonment of a rapper convicted of insulting the Spanish monarchy and praising terrorist violence were marred by rioting for a third successive night on Thursday.

The plight of Pablo Hasel, who began this week to serve a nine-month sentence in a north-eastern prison, has triggered a heated debate over the limits of free speech in Spain and a political storm over the use of violence by both the rapper’s supporters and the police.

The ruling coalition’s junior partner, the far-left United We Can (Unidas Podemos) party, on Thursday filed a petition for a “total pardon” for Hasel and another rapper, Valtonyc, who fled to Belgium in 2018 to avoid trial on charges of “glorifying” terrorism.

But potentially deepening the tension, court authorities in the north-eastern Catalonia region announced Hasel lost a recent appeal and is looking at an additional prison sentence of two and a half years for obstructing justice and assault in 2017. The sentence can go to appeal again before the country’s Supreme Court.

Like the two previous nights, the protests began on Thursday with large gatherings in several cities that were, at first, mostly peaceful.

In Catalonia’s regional capital, Barcelona, hundreds sang songs, rapped and shouted “Pablo Hasel, freedom!” and “Spanish media, manipulators!” at a central square.

Dozens then broke off from the main group to set alight a barricade of trash containers and a construction skip that blocked a main city artery, hurling stones, bottles and other objects at riot police.

There were moments of tension as flames threatened to extend to nearby buildings before firefighters arrived.

In the eastern coastal city of Valencia, police used batons to disperse protesters and arrested at least eight people, according to the Spanish government’s regional delegation.

On Friday, prime minister Pedro Sanchez said: “Violence is an attack on democracy, and the government will take a stand against any form of violence to ensure people’s safety.”

Interior minister Fernando Grande-Marlaska also stepped into the row, thanking police for their efforts and saying they would continue to “guarantee the rights and freedoms of all society against a minority whose misguided idea of rights makes them have recourse to violence”.