Barack Obama has become the first serving US president to visit Hiroshima since the World War Two nuclear attack.
Mr Obama said the memory of 6 August 1945 must never fade, but did not apologise for the US attack - the world's first nuclear bombing.
Mr Obama spoke to two survivors and in an address called on nations to pursue a world without nuclear weapons.
At least 140,000 people died in Hiroshima and another 74,000 three days later in a second bombing in Nagasaki.
'Best of friends'
Mr Obama first visited the Hiroshima Peace Memorial Museum before walking to the Peace Memorial Park, accompanied by Prime Minister Shinzo Abe.
Both men stood in front of the eternal flame. Mr Obama laid a wreath first, followed by Mr Abe.
"Death fell from the sky and the world was changed," Mr Obama said in his address, noting that the bombing had shown that "mankind possessed the means to destroy itself".
Mr Obama said the memory of Hiroshima must never fade: "It allows us to fight complacencies, fuels our moral imagination and allows us to change."
Of nuclear weapons, he said: "We must have the courage to escape the logic of fear and pursue a world without them."
Mr Obama then spoke to two survivors, hugging 79-year-old Shigeaki Mori.
"The president gestured as if he was going to give me a hug, so we hugged," Mr Mori said.
