"Regrettably, things have not improved, and in some places, things have gotten worse."

"A voice of protest is needed," especially when the lives of children were at stake.

The missing students' case has caused consternation in Mexico and attracted enormous international attention.

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."