“71 years ago, death fell from the sky and the world was changed,” the president said after laying a wreath, as he became the first sitting US leader to visit the site.
Obama looked sombre as he offered the wreath, lowering his head and pausing for a moment with his eyes closed before withdrawing and watching Japanese Prime Minister Shinzo Abe lay his flowers.
The bomb “demonstrated that mankind possessed the means to destroy itself”.
“Why did we come to this place, to Hiroshima? We come to ponder a terrible force unleashed in the not so distant past. We come to mourn the dead,” he said.
“Their souls speak to us, they ask us to look inward, take stock of who we are,” he said.
“Technological progress without equivalent progress in human institutions can doom us. The scientific revolution that led to the splitting of the atom requires a moral revolution as well.
“This is why we come to this place, we stand here, in the middle of this city and force ourselves to imagine the moment the bomb fell.
“We force ourselves to feel the dread of children confused by what they see. We listen to a silent cry.”
President Obama made history Friday as he readied to visit Hiroshima — and become the only sitting US leader to set foot on ground that was seared by the world’s first nuclear attack.
The trip comes more than seven decades after the Enola Gay bomber dropped its deadly atomic payload, dubbed “Little Boy”, over the western Japanese city.
The bombing claimed the lives of 140,000 people, some of whom died immediately in a ball of searing heat, while many succumbed to injuries or radiation-related illnesses in the weeks, months and years afterwards.
A second nuclear bomb was dropped on Nagasaki three days later.
Coming in Obama’s final year in office, the visit also marks seven years since he used his trademark soaring rhetoric to call for the elimination of atomic arms in a landmark speech in Prague that helped him win the Nobel Peace Prize.
And while the world today appears no closer to that lofty vision, Obama is expected to use the symbolism of his presence in Hiroshima to highlight a push for peace.
Anticipation was high in Hiroshima, where crowds of Japanese and visiting foreigners gathered near the Hiroshima Peace Memorial Park where Obama was to appear.
“We welcome President Obama,” said 80-year-old Toshiyuki Kawamoto. “I hope this historic visit to Hiroshima will push for the movement of abolishing nuclear weapons in the world.”
Japanese and American flags flew on the street in front of the site, with a city official saying it was the first time the Stars and Stripes had been raised there.
Obama, earlier arrived at a US military base west of Hiroshima to address troops after attending a Group of Seven summit elsewhere in Japan.
The cenotaph lies in the shadow of a domed building, whose skeleton has been left standing in silent testament to the victims of the first ever nuclear attack.
He told American troops at a base in Iwakuni in the west of the country that visiting Hiroshima was a chance to “honour” the memory of all who died in the war.
“It’s a testament to how even the most painful divides can be bridged,” he said. “How two nations can become not just partners but the best of friends.”
But Obama also said that the two countries were “reaffirming one of the greatest alliances in the world.”