After the 1976 coup, Marshall showed his usual stubbornness of purpose and willingness to challenge authority as he dedicated more of his time to human rights work. Throughout the military dictatorship, which came to be known as El Proceso (1976-1983), Marshall was approached day and night by relatives of the disappeared who had nowhere else to turn.
Marshall frequently visited political prisoners in his capacity as rabbi and also worked alongside many leading human rights advocates and associations. He was in constant contact with groups such as Madres de Plaza de Mayo and El Servicio de Paz y Justicia (SERPAJ), founded by 1980 Noble Peace Prize winner Adolfo Pérez Esquivel, and he was also a founding member of the Asamblea Permanente por los Derechos Humanos (Permanent Assembly for Human Rights or APDH).
Throughout El Proceso, Marshall experienced the anti-Semitism of the military firsthand. It was not uncommon to see Nazi symbols and images of Hitler hanging in the prisons. “I was left nude in the middle of the winter on the patio of one of the prisons [Villa Devoto, in Buenos Aires],” Marshall said in an interview, “while the prison director walked by and made sure I heard the fact that ‘This Jew is going to walk in one day through the front door, and he’s going to go out in a coffin through the back door.’”
Because he was an outspoken human rights advocate, Marshall and his family were subject to constant death threats. Their family telephone was tapped, their house was probably bugged, and Marshall was followed wherever he went by plainclothes police officers.