He was forced into exile in Caracas following the coup d'état of 11 September 1973 that overthrew President Salvador Allende. That did not prevent him, however, from founding the Chilean Human Rights Commission "(Comisión Chilena de Derechos Humanos)" in 1978, an agency that worked to defend human rights during the "de facto" government of General Augusto Pinochet. In that capacity, he defended several human rights cases, including cases involving Chilean exiles or the murder of Orlando Letelier. In August 1976 Castillo Velasco and Eugenio Velasco Letelier (es), were accused of posing a threat to "national security" and were expelled to Buenos Aires. While in exile he tried a few times to return to his country but the Junta did not allow him.