"To speak in this place of horror, in this place where unprecedented mass crimes were committed against God and man, is almost impossible, and it is particularly difficult for a Christian, for a pope from Germany," Benedict said in an address at the camp's Birkenau annex.
At the site where Nazis exterminated more than a million people, most of them Jews, he said: "In a place like this, words fail."
The 79-year-old German born pontiff was on the final day of a four-day visit to the homeland of his predecessor John Paul II.
Benedict said he had come to Auschwitz "as a son of the German people. "I could not fail to come here. I had to come," he said.
"It is a duty before the truth and the just, due of all who suffered here, a duty before God, for me to come here as the successor of Pope John Paul II and as a son of the German people."
Bowl of candles
A solemn Benedict, his hands clasped in prayer, had entered the camp gates on foot, 20 metres in front of his cardinals, as church bells rang in the southern town of Oswiecim, the Polish name for Auschwitz.
After placing a bowl containing a lighted candle at the camp's execution wall, where the Nazis summarily shot thousands of inmates, he moved along a line of 32 camp survivors waiting to meet him.
Benedict clasped the hands of the first, a woman wearing the striped scarf that Polish political prisoners wore at the camp.
As the white-cassocked pontiff moved along the line, an elderly Polish man kissed him on both cheeks, a gypsy survivor of the camp pressed the pope's hand to his lips.
Henryk Mandelbaum, 83, wearing the distinctive striped cap of the
Sonderkommando, Jewish prisoners who emptied the gas chambers where their fellow Jews perished, bowed to kiss the papal ring.
Inside the grim barracks, Benedict prayed before a lighted candle in the cell where Catholic priest Maximilian Kolbe died after taking the place of a prisoner the Nazis had sentenced to death by starvation.
A brilliant rainbow lanced a leaden sky as Benedict paused before each of the 22 plaques at the Birkenau annex's International Monument to the Victims of Fascism.
The plaques commemorate people from various countries exterminated at the camp.
Rabbi’s prayers
Poland's Chief Rabbi Michael Schudrich sang the Kaddish, a Jewish prayer of mourning, while musicians played a haunting Jewish lament.
"They would stir our hearts profoundly if we remembered the victims not merely in general, but rather saw the faces of individual persons who ended up here in this abyss of terror," said the pope.
In his speech, Benedict paid tribute to the Jewish inmates murdered at the camp. "The place where we are standing is a place of memory, and at the same time the place of the Shoah," he said.
"The rulers of the Third Reich wanted to crush the entire Jewish people, to cancel it from the register of the peoples of the earth."
However, his declaration that six million Poles lost their lives to the Nazis, while failing to mention that three million of them were Jews, disappointed some Jews, including Israel's ambassador to Poland David Peleg.
"I think it is important to make the distinction between the Jews who were murdered and others. I think that there is a uniqueness to the Jewish victims because of the Germans' Final Solution.
"With all the respect and tragedy for the murder of others, this is one thing we think must be emphasised," he said.
The pontiff also absolved the German people from blame for the outrages committed against the Jews.
The pope described himself as "the son of a people over which a ring of criminals rose to power."
The German people had been "used and abused" as an instrument of Hitler's "thirst for destruction and power," he added.
The visit sealed a trip in which Benedict traced the footsteps of his spiritual mentor John Paul II in this bastion of Roman Catholicism, before flying back to Rome nearly two hours behind schedule.