Barack Obama has become the first serving US president to visit Hiroshima since the World War II nuclear attack on August 6, 1945.
He says the memory must never fade, but he did not apologise for the attack -- the world's first nuclear bombing.
"Seventy-one years ago, on a bright cloudless morning, death fell from the sky and the world was changed," Mr Obama said in opening his speech at the memorial.
"Technological progress without an equivalent progress in human institutions can doom us."
Mr Obama called for a world without nuclear weapons and he spoke to two survivors.
"I held his hand, and we didn't need an interpreter," survivor Sunao Tsuboi, 91, said.
"I could understand what he wanted to say by his expression."
At least 140,000 people died in Hiroshima and another 74,000 three days later in the bombing of Nagasaki.