"What on earth is happening in the hearts of men? What on earth is happening in the heart of humanity?" he said to tens of thousands of people in St Peter's Square on the day the Roman Catholic Church celebrates its World Day of Peace.
"It's time to stop!" he said, departing from his prepared text.
Francis was speaking from the window of the Apostolic Palace overlooking St. Peter's Square that popes traditionally use for their addresses.
Unlike his predecessors, Francis has not used the spacious apartment behind the window since his election in March, shunning pomp and preferring instead to live in small quarters in a Vatican guest house.
"Everyone must be committed to building a society that is truly just and caring," he told the crowd on Wednesday, acknowledging the many peace banners and blue balloons held aloft.