In addition to his academic responsibilities, Berrigan became active in the Civil Rights Movement. He marched for desegregation and participated in sit-ins and bus boycotts. His brother Daniel wrote of him:
From the beginning, he stood with the urban poor. He rejected the traditional, isolated stance of the Church in black communities. He was also incurably secular; he saw the Church as one resource, bringing to bear on the squalid facts of racism the light of the Gospel, the presence of inventive courage and hope.
Berrigan was first imprisoned in 1962/1963. During his many prison sentences he would often hold Bible study class and offer legal educational support to other inmates. As a priest, his activism and arrests met with deep disapproval from the leadership of the Catholic Church and Berrigan was moved to Epiphany Apostolic College, the Josephite seminary college in Newburgh, New York, but he continued his protests. Working with Jim Forest, in 1964 he founded the Catholic Peace Fellowship in New York City. He was moved again to St. Peter Claver Parish in West Baltimore, Maryland, from where he started the Baltimore Interfaith Peace Mission, leading lobbies and demonstrations.
“We Christians forget (if we ever learned) that attempts to redress real or imagined injustice by violent means are merely another exercise in denial - denial of God and her nonviolence towards us, denial of love of neighbor, denial of laws essential to our being.”
-- Philip Berrigan