Pope Francis said human "cruelty did not end in Auschwitz" and that similar atrocities are being inflicted in war zones across the world today, citing prisoners who are kept in inhuman conditions and tortured.
Francis visited the former Nazi death camp of Auschwitz-Birkenau on Friday, expressing his sorrow there in contemplative silence and prayer.
Only hours later did he finally speak out about his feelings as he addressed pilgrims from a window of the archbishop's residence in Krakow.
He said: "How much pain? How much cruelty? Is it possible that we humans created in God's image are capable of doing these things? ...Cruelty did not end in Auschwitz, in Birkenau."
The pope continued: "Many prisoners are tortured just to make them talk. It's terrible. Today, there are men and women in overcrowded prisons. They live - forgive me - like animals. Today, there is this cruelty. We say, yes, there we saw the cruelty of 70 years ago, how people died being shot or hanged or with gas.
"Today in many parts of the world where there is war, the same thing is happening."
The pope had earlier paid a sombre and largely silent visit to the Nazi death camp of Auschwitz-Birkenau, becoming the third consecutive pontiff to make the pilgrimage to the place where Adolf Hitler's forces killed more than a million people.
Francis entered the camp on foot, walking slowly in his white robe beneath the notorious gate at Auschwitz bearing the words Arbeit Macht Frei (Work Will Set You Free).
After Auschwitz he moved to nearby Birkenau, where people were murdered in factory-like fashion in gas chambers.
It was a contemplative and private visit of nearly two hours that Francis passed in silence, except for a few words he exchanged with camp survivors and Holocaust rescuers.
Vatican and Polish church officials had explained that he wanted to express his sorrow in silence at the site, mourning the victims in quiet prayer and meditation.
However, he did express his feelings, writing in the Auschwitz memorial's guest book in Spanish: "Lord, have pity on your people. Lord, forgive so much cruelty."
As an Argentinian, he is the first pope to visit Auschwitz who did not live through the brutality of the Second World War on Europe's soil.
Both of his predecessors had a personal historical connection to the site, with the first, John Paul II, hailing from Poland and himself a witness to the suffering inflicted on his nation during the German occupation. His successor Benedict XVI, who visited in 2006, was a German who served in the Hitler Youth for a time as a teenager.
Francis prayed silently for more than 15 minutes before meeting several survivors of the camp, greeting them one by one, shaking their hands and kissing them on the cheeks. He then carried a large white candle to the Death Wall, where prisoners were murdered.
At the dark underground prison cell that once housed St Maximilian Kolbe, a Polish Catholic friar who sacrificed his own life during the war to save the life of another man, Francis prayed again. A few shafts from a tiny window were the only light cast on the white-clad figure.
He then travelled the two miles to Birkenau, the vast satellite camp where the Nazis murdered Jews, Roma and others from across Europe.
Invited guests, among them camp survivors and Christian Poles who saved Jews during the war, stood in respect as the Pope arrived, his vehicle driving parallel to the rail tracks once used to transport victims to their deaths.
When Francis arrived, the hundreds of guests applauded. He slowly observed each of the memorial plaques in the 23 languages used by the inmates.
Poland's chief rabbi, Michael Schudrich, then recited in Hebrew Psalm 130, which starts: "From the depths I have cried out to you, O Lord."
Francis clasped his hands and bent his head as the psalm was read first by the rabbi and then by a priest in Polish.
The visit to Auschwitz came on the third day of a five-day visit to Poland that includes meetings with young pilgrims taking part in World Youth Day, a global youth celebration.