Reparations, he said, "can mean a whole bunch of different things. You can come up with policies that take into account what slavery meant then, what it means now."

The pope's appeal came in the wake of a surge in violence in Syria. Government-led air raids reportedly bombed hospitals and other medical centers, killing patients and medical personnel.

The event was sponsored by the Catholic charity Aid to the Church in Need, which wanted to "draw attention to the dramatic issue of anti-Christian persecution."

One idea being considered is a detailed map laying out "safe zones" where civilians and members of moderate opposition groups covered by the truce.

In Greece, the faithful attended Easter Mass holding candles lit with "Holy Fire" from the Church of the Holy Sepulchre in Jerusalem.

Berrigan undoubtedly stands among the most influential American Jesuits of the past century.