Seated on a bench near the gate to the Auschwitz death camp site in Poland, Pope Francis prayed in silence in tribute to the 1.5 million people, most of them Jews, killed there by Nazi occupiers during World War Two.
The third pope to visit Auschwitz and the first not to have lived through the war in Europe, he entered the camp by foot, passing through iron gates under the infamous sign reading "Arbeit Macht Frei", German for "Work Sets You Free".
Visibly moved by the sight of the wooden guard towers, barbed wire fences and inmate barracks, he sat in silent prayer for about 15 minutes.
The pope said before the trip that he had decided to make no statement as silence was the best way to honour the dead.
He later spent a few minutes quietly greeting 11 Auschwitz survivors, kissing each of them on both cheeks.
One man gave the pope a picture of himself surrounded by other emaciated inmates in a bunk, and asked the pope to sign it.
The 79-year-old Argentine-born pontiff then proceeded to walk through the barely lit corridors of the drab, brick building of Auschwitz Block 11, which had housed prisoners selected for special punishment.
With aides using small flashlights to light his way, Francis visited the underground cell where Franciscan monk Maksymilian Kolbe was killed after offering his life to save a Polish man whom camp handlers had picked to die of starvation.
Just outside the cell, in Auschwitz's commemorative book, Francis wrote in Spanish: "Lord, have mercy on your people. Lord, forgiveness for so much cruelty".
German occupation forces set up the Auschwitz-Birkenau camp during World War Two in Oswiecim, a town around 70 km from Poland's second city, Krakow, in the country's south.
Between 1940 and 1945 Auschwitz developed into a vast complex of barracks, workshops, gas chambers and crematoria.
Pope Francis, who has made many strong condemnations of anti-Semitism, was driven past ruins of crematoria that the Nazis blew up before the camp was liberated by the Soviet Red Army.
He listened silently as Poland's chief rabbi, Michael Schudrich, sang from Psalm 130 and a priest read its words in Polish, just metres away from the end of the infamous single rail track where cattle cars brought hundreds of thousands of prisoners to the camp.
He paused at each of 23 plaques, each reading in a different language: "For ever let this place be a cry of despair and a warning to humanity."