By Alastair Wanklyn – Tokyo, Japan
In a park near where the atomic bomb fell, several hundred survivors and the relatives of those who died gathered to remember the victims.
They stood in prayer as a bell marked the moment the bomb fell.
Japan's Prime Minister Shinzo Abe told mourners that Japan will work as a bridge between nations to abolish nuclear weapons.
The mayor of Hiroshima said the city's post-war recovery made the city a symbol of peace. Mayor Kazumi Matsui urged civil society worldwide to reject nationalism and unite against all threats.
Around 140,000 people died in the explosion, or from injuries such as radiation burns, in the weeks that followed. Three days after destroying Hiroshima, the United States detonated a second bomb over the city of Nagasaki.
Calls for nuclear disarmament
Earlier this week, the mayor of Nagasaki said that, although nations blow hot and cold on nuclear disarmament, the people of Hiroshima and Nagasaki will always speak up.
Nagasaki Mayor Tomihisa Taue said it's important that those small voices continue to remind the world of the inhumanity of nuclear weapons.
Meanwhile, in his address on Thursday, the mayor of Hiroshima reminded the mourners of the encouragement Pope Francis gave to them during his visit to Hiroshima in November last year.
During his visit, the Pope said the possession of atomic weapons is immoral. Future generations will judge us, he said, if we talk of peace but do not act towards disarmament. He said the possession of weapons gives a false sense of security, sustained by fear and mistrust.
Bishops warn of threats to peace
Last month the Catholic Bishops Conference of Japan released a message marking 75 years since the end of World War II.
The bishops noted new threats to global peace, including a new Cold War, instability in East Asia, and a global environmental crisis.
The bishops also spoke of the urgency of denuclearization. Last year they urged Japan's government to sign the United Nations Treaty on the Prohibition of Nuclear Weapons.
More than 80 countries have signed the treaty. But Japan has not. It relies on the nuclear protection of the United States.