By Chris Hogg
BBC News, Hiroshima
They started to come two hours before the ceremony got under way, streaming over the bridges and into the Peace Park.
The elderly prayed quietly or shared memories with younger people
It was early morning, before 0600, but already it was hot.
The bright, sunny August morning with barely a cloud in the sky was almost the same conditions as those of that day 60 years before.
Yuri Kitamura, now 79, was just 19 when the bomb exploded above the city, killing tens of thousands of Hiroshima's people.
She has come back to the park to remember.
"It's been many years and months to get to where we are today," she said.
"But it doesn't matter how long it has taken. The memories of the day come back vividly. They still cause me pain."
Alan James and John Larkin, who had come here from Corby in north Lancashire, told me there was a feeling of serenity in the park.
Tom, from Canada, said there was no alternative to world peace. "I want to spread the word back home," he said.
Monks chanted by a mound containing the ashes of the dead
Barra McKenzie, from New Zealand, said it would be so much more impressive if other countries could promote peace in the same way that Japan does.
"It was very moving. It's a hot day and there's not much shade but it really brings it home to you. It's amazing that all these Japanese people are actively promoting peace."
Around the edges of the main ceremony, different rituals and commemorations were taking place.
Shinto priests in their flowing white robes celebrated an elaborate ritual in front of the mound which contains the ashes of 70,000 of those who died.
On the other side of the mound, Buddhist monks were chanting.
Occasionally, an elderly man or woman would step forward and gently place some flowers on the makeshift shrine.
At 0815 the peace bell rang out across the park and instantly people stopped where they were, some even in the middle of the road, and bowed their heads.
Thousands gathered in the hot sun to remember the bomb victims
On one side of the park, a small group of demonstrators protested against the presence of Japanese Prime Minister Junichiro Koizumi here. Shouting through the silence, they were watched closely by the police.
After the main ceremony finished, many people stayed around, soaking up the atmosphere in the park.
It got hotter and hotter as the morning wore on, but no-one seemed to care.
Many, it seemed, felt like this was the right place to be today. It was almost as if they wanted to stay to share the memories of lost loved ones.
Old men showed children their yellowing photographs. Others just sat quietly under the trees, lost in their own thoughts.
And from time to time, the peace bell rang out still, as people clambered up on to the pedestal to ring it for themselves and signal their desire to rid the world of nuclear weapons.