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In 1934 Carrillo was elected as secretary-general of the FTS and became editor of its newspaper, Renovación. He used his position to support the left-wing of the PSOE led by Francisco Largo Caballero.
Carrillo's support for the Asturian rising in October 1934 led to his arrest. Along with other PSOE leaders Carrillo was imprisoned in Madrid.
Carrillo was released from prison after the Popular Front victory in February, 1936. He visited the Soviet Union and after his return worked closely with Comintern agents in Spain. This included turning the FJS into an communist organization. Carrillo announced his membership of the Communist Party (PCE) on 6th November 1936.
After the Nationalist Army victory, Carrillo fled to France. On the outbreak of the Second World War Carrillo moved to the Soviet Union where he became general secretary of the Spanish Communist Party (1960-1982) in exile.