BARCELONA — Off a leafy boulevard in Barcelona sit the headquarters of Omnium Cultural, an organization known in Spain as much for its literary prizes as for its dreams of an independent republic in Catalonia.
But its president, Jordi Cuixart, is nowhere to be found: For the last three and a half years, he has lived in a prison cell.
To the Spanish authorities, Mr. Cuixart is a dangerous criminal, convicted of sedition for leading a rally at a time when he and other separatist leaders were seeking to set up a breakaway state in the northeastern region of Catalonia. Yet to his supporters, and in the eyes of many foreign countries, he is a political prisoner sitting in the heart of Europe.
“They want us to change our ideals,” Mr. Cuixart said, speaking through a thick pane of glass in the prison visitors’ section on a recent afternoon.
More than three years have passed since the Catalonian independence movement nearly tore Spain apart, and the…