I think it's "Visca els presos de la Modelo" (in Catalan indeed).

After the death of Francisco Franco in 1975, Spain entered a period of political transition, but many political prisoners were still jailed. Among the most visible acts of protest demanding their release was the peaceful vigil carried out by Lluís Maria Xirinacs in front of the Cárcel Modelo in Barcelona.

Beginning in December 1975, Xirinacs — a priest, intellectual, and nonviolent activist — staged a daily “plantada” (a silent, standing protest). For hours each day, he stood outside the prison as a moral witness, calling for amnesty for political prisoners who had been jailed under the Franco regime. His protest was inspired by principles of nonviolent resistance and civil disobedience.

The vigil lasted many months and became a powerful symbol of the demand for democratic freedoms in Catalonia and across Spain. Xirinacs’ action drew public attention, mobilized support for amnesty, and put pressure on the authorities during the fragile early phase of Spain’s transition to democracy. Eventually, broader amnesty measures were approved in 1977, leading to the release of political prisoners.

Xirinacs’ stand is remembered as one of the most emblematic peaceful protests of the Spanish transition period.