The crowd stood in the heavy summer heat as cicadas shrilled and the Peace Bell rang.
Thousands bowed their heads in prayer at 8:15am, the exact time the bomb exploded on August 6, 1945.
The Hiroshima bombing, which killed about 140,000 people by the end of the year, was followed by the atomic bombing of Nagasaki on August 9, 1945, which killed about 40,000 people instantly.
The war ended on August 15.
Hiroshima Mayor Kazumi Matsui used the anniversary to call for nuclear weapons to be abolished, and for the creation of security systems that do not rely on military might.
"As long nuclear weapons continue to exist, there is no way to know who and when its next victims will be. And if the bomb is dropped, the damage will reach indiscriminately beyond borders."
Japanese Prime Minister Shinzo Abe also called for an end to nuclear weapons.
"Japan intends to renew its efforts to bring about a world without nuclear weapons, with the cooperation of both nuclear powers and the non-nuclear powers. And that resolve translates to us proposing a new draft resolution at the United Nations in the fall on nuclear disarmament."
The impact of Hiroshima and Nagasaki lives on.
Thousands of people continue to be treated for cancer and other related health conditions as a result of the bombs dropped on both Hiroshima and Nagasaki.
In 2014 Red Cross hospitals in Japan treated just over 10,000 Hiroshima and Nagasaki atomic bomb survivors.
Robert Tickner is the Chief Executive of the Australian Red Cross.
He says the impact has been far reaching.
"Well we still see cancers impacting on people who were either in their mother's womb at the time of the bomb or young children in hugely disproportionate numbers to the wider community. This is 70 years down the track after hundreds of thousands of people of course have passed away in the intervening years."
The Red Cross says there are more than 16,000 nuclear weapons around the globe and it's calling on world Governments to act to prohibit and eliminate nuclear weapons once and for all.
Mr Tickner says 113 countries around the world have committed themselves to working towards some kind of convention or treaty to outlaw nuclear weapons.
He adds that what's needed now is a process to start which would make this a tangible reality.
And he wants to see Australia do more to support this process.
"Well sadly we've been trying over a period of time to get support from both sides of politics this is an issue which ought to be above politics. We're really waiting to see what the parliamentarians and particularly our leaders are going to say 70 years down the track."
