Obama, who is presently in Japan to attend the G7 summit, is the first American president to visit Hiroshima, seven decades after its bombing.
US President Barack Obama on Friday visited Hiroshima memorial to honour those who lost their lives in the world's very first nuclear bombing by the United States in 1945 during World War II.
Looking somber, Obama accompanied by Japan's Prime Minister Shinzo Abe appeared at the Hiroshima memorial and laid a floral wreath at the cenotaph.
Speaking on the occasion, Barack Obama paid tribute to the victims of the world's first nuclear attack during a historic visit to Hiroshima.
"71 years ago, death fell from the sky and the world was changed," the president said. The bombing "demonstrated that mankind possessed the means to destroy itself," said Obama, who today became the very first serving US president to have visited the Hiroshima memorial site seven decades after its bombing.
"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.
Barack Obama 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."
Obama greeted ageing survivors of US nuclear attacks, embracing one elderly man who appeared overcome with emotion.
Obama did not issue any apology, as he had earlier said he would not. But, he said that he would honour all those who lost their lives in World War II.
Analysts are terming the US President's visit to Hiroshima as a significant world development, as Obama told the Japanese media earlier that ‘even former adversaries can become the strongest of allies.'
In 1945, Japan witnessed the world's very first nuclear bomb attacks. Hiroshima was bombed on August 6, 1945, killing at least 140,000 people, while another one of its cities Nagasaki saw a similar attack two days later that killed 74,000 more.
Protest against Obama's visit in Hiroshima
Earlier, students, labour unions and anti-nuclear activists demonstrated in Hiroshima protesting against Barack Obama's visit to Hiroshima.
The visit sparks debate
The visit has stirred debate, with critics accusing both sides of having selective memories and pointing to paradoxes in policies relying on nuclear deterrence while calling for an end to atomic arms.
The two governments hope Obama's tour of Hiroshima will highlight a new level of reconciliation and tighter ties between the former enemies.
A majority of Americans see the bombings as having been necessary to end the war and save lives, although some historians question that view. Most Japanese believe they were unjustified.
"I'm coming, first and foremost, to remember and honour the tens of millions of lives lost during the Second World War. Hiroshima reminds us that war, no matter the cause or countries involved, results in tremendous suffering and loss, especially for innocent civilians," Obama said in written responses to questions published in the Asahi newspaper on Friday.
"I will not revisit the decision to use atomic weapons in Hiroshima and Nagasaki, but I will point out that Prime Minister (Shinzo) Abe and I coming to Hiroshima together shows the world the possibility of reconciliation - that even former adversaries can become the strongest of allies," Obama told the Asahi.
Previously, former US presidents Jimmy Carter and Richard Nixon visited Hiroshima's Peace Memorial Park, but neither was in office at the time of their visits.