Pedro was born in the freguesia of Samouco in the municipality of Alcochete, Portugal, on 8 November 1918.[1] In 1934, Pedro, who was 15-years old, was arrested for the first time for participating in a general strike.[1] He joined the Portuguese Communist Party (PCP) later in the 1930s, where he met the party's leader, Álvaro Cunhal.[1] In 1936, Edmundo Pedro was arrested again and sent to the Tarrafal prison camp in Cape Verde alongside his father, Gabriel Pedro, another opponent of the Salazar government.[1]
In 1945, while still in prison, Pedro broke with the PCP and left the party.[1] He was released from Tarrafal in 1946 and returned to mainland Portugal.