Japan marked the 70th anniversary of the atomic bombing of Hiroshima with Mayor Kazumi Matsui urging world leaders to step up efforts towards making a nuclear weapons free world.
Tens of thousands of people stood for a minute of silence at 8.30am local time at the ceremony in Hiroshima's peace park near the epicentre of the 1945 attack, marking the moment of the atomic blast.
Then dozens of doves were released as a symbol of peace.
The US bomb, "Little Boy," the first one used at war, killed 140,000 people, and a second bomb "Fat Man," dropped over Nagasaki three days later, killed another 70,000, prompting Japan's surrender in the Second World War.
Mr Matsui called the nuclear weapons "the ultimate inhumanity and the absolute evil" that must be abolished.
He criticised nuclear powers for keeping them as threats to achieve their national interests. He said the world still bristles with more than 15,000 nuclear weapons.
He renewed an invitation to US president Barack Obama and other world leaders to visit Hiroshima and Nagasaki to see the scars themselves.
"President Obama and other policymakers, please come to the A-bombed cities, hear the hibakusha (survivors) with your own ears, and encounter the reality of the atomic bombings," he said.
"Surely, you will be impelled to start discussing a legal framework, including a nuclear weapons convention."
The anniversary comes as Japan is divided over Prime Minister Shinzo Abe's push to pass unpopular legislation to expand the country's military role internationally.
it comes a year after his cabinet's decision to loosen Japan's war-renouncing constitution by adopting a new interpretation of it.
Mr Abe said that as the sole country to face a nuclear attack, Japan had a duty to push for the elimination of nuclear weapons. He pledged to promote the cause through international conferences to be held in Hiroshima later this month.
With the average age of survivors now exceeding 80 years for the first time this year, passing on their stories is considered an urgent task.
There were 5,359 "hibakusha" who died over the past year, bringing the total death toll from the Hiroshima bombing to 297,684.
The sea of people who attended the ceremony included US Ambassador Caroline Kennedy and representatives from more than 100 countries, including Britain, France and Russia.
"Little Boy," dropped from the Enola Gay, a B-29 bomber, destroyed 90% of the city and killed an estimated 140,000 people, including those who succumbed to injuries and radiation sickness in the ensuing weeks.
A "black rain" of radioactive particles followed the blinding blast and fireball, and has been linked to higher rates of cancer and other radiation-related diseases among survivors.