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Pablo Hasel had until Friday last week to surrender and serve his nine-month sentence, which has caused an uproar across Spain and led the government to announce it would ease restrictions on free speech, even if not in his particular case.
"We are trying to raise awareness of what is happening ... This is a very serious attack on our freedoms, not just on me," Hasel, who is known for his radical leftist views, told Reuters.
Hasel's lyrics and tweets, which included references to banned guerrilla groups, compared a court to Nazis and called former king Juan Carlos a mafia boss, were found by a court to have encouraged violence and insulted the monarchy.
He and around 20 supporters used a chain with a padlock to shut the main entrance to one of the buildings of Lleida university in the region of Catalonia where they took refuge. Police had yet to show up.
"We don't know if they are coming in half an hour or a few days," he said, adding that he faced two years behind bars because he was not planning to pay the fine that was part of his sentence.
In a sign of support, local artist Cinta Vidal has painted graffiti in a town near Barcelona depicting a singing Hasel being painted over by former king Juan Carlos with a roll brush.
"It is not acceptable that an artist should go to prison for expressing his ideas," Vidal said.