Over 140,000 people died there in August of 1945
Barack Obama has made an historic visit to Hiroshima, a first for a sitting US President.
At least 140,000 people died in the Japanese city in August of 1945 - as a result of the world's first nuclear bombing.
Two days, later a second nuclear bomb was dropped on Nagasaki, killing another 74,000 people.
Mr Obama, who is in Japan for the G7 summit, said his visit was "a testament to how even the most painful of divides can be bridged".
He also said the US would not be apologising for the attack, which hastened the end of the Second World War.
Many historians say the strike saved thousands maybe millions of lives, by convincing Japanese leaders to surrender without the need for a bloody invasion of the home islands.
"Seventy-one years ago, death fell from the sky, and the world was changed," Obama said.
"A flash of light and a wall of fire destroyed a city."
Obama is the first sitting US President to visit the Japanese city, which was the scene of the first - and last - atomic bomb deployed by any nation in combat.
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 a number of survivors and in an address called on nations to pursue a world without nuclear weapons, bringing global attention both to survivors and to his unfulfilled vision of a nuclear-free world.
Alongside Japanese Prime Minister Shinzo Abe, Mr Obama laid a wreath at the Peace Memorial Park in the city and honoured all those who died during World War Two.
Hideki Asari, from the Japanese Embassy, hopes the visit will make a difference.
"We saw President Obama talking to, talking very tentatively to, survivors. And [he] embraced one of them.
"It was a very moving scene to watch. It is very important to strive for a world free of nuclear weapons."
Mr Obama, who won a Nobel Peace Prize in 2009 on the back of his promotion of nuclear non-proliferation, used a speech at the memorial site to reaffirm his commitment to reducing global stockpiles of nuclear weapons.
He said: "The scientific revolution that led to the splitting of the atom requires a moral revolution as well.
"That 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.
"We have a shared responsibility to look directly into the eye of history, and ask what we must do differently to curb such suffering again.
"The memory of the morning of August 6, 1945, must never fade. We must have the courage to escape the logic of fear and pursue a world without them."
Mr Obama did not apologise for the bombing, but acknowledged the devastating toll of the conflict.
Thousands of people were killed instantly when the atomic bomb was dropped on the city on 6 August, 1945.
Some 145,000 died by the end of that year.
The city of Nagasaki was hit by a second nuclear bomb on 9 August, 1945, and Japan surrendered six days later.
Ahead of the visit, protesters had gathered outside the peace park to demand an apology from the US President.
Mr Obama spoke briefly with five survivors of the atomic bombing who had attended the ceremony along with their families.
He also signed a guest book at the memorial park, writing: "We have known the agony of war. Let us now find the courage, together, to spread peace, and pursue a world without nuclear weapons."
Mr Abe said the visit hailed a new chapter of reconciliation between the US and Japan.
The visit has been heavily scrutinised in the US and in Japan.
Critics in the US claimed even making the visit could be seen as a form of apology for the bombing, while many in Japan were hoping for a formal apology.