Mr. Obama also met survivors of the attack, which killed at least 140,000 people - but did not apologise for the bombing.
Here's our US correspondent Lorna Shaddick.
IN: A crowd of...
OUT: SOC
DUR: 42 seconds
POT: 32 seconds
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After laying his wreath, Mr. Obama described what happened in Hiroshima when the world's first nuclear weapon was deployed:
OUT: ...to destroy itself
DUR: 22 seconds
President Obama said what happened at Hiroshima must never be forgotten:
IN: The memory of...
OUT: ...a common humanity
DUR: 25 seconds
The US President said he hoped the bombings of Hiroshima and Nagasaki can be taken as a lesson for future generations
IN: But today, the...
OUT: ...own moral awakening
DUR: 24 seconds