Speaking at the Peace Memorial Park near Ground Zero, Hiroshima Mayor Kazumi Matsui said countries should put aside their differences and come together to overcome both man-made and natural challenges, reports The Japan Times newspaper.
A moment of silence was observed at 8.15 a.m., the exact time of the bombing on August 6, 1945.
But this year's ceremony was drastically scaled down due to a recent spike in the number of coronavirus in Japan.
There were only about 880 seats, less than one-10th of the usual number, and scrapped sections allocated for general admission.
Matsui further said that "the people of the world must unite to achieve nuclear weapons abolition and lasting world peace".
Also at the ceremony, Prime Minister Shinzo Abe said "each country must step up efforts to "remove a sense of mistrust through mutual involvement and dialogue", amid the severe security environment and widening differences between nations' positions on nuclear disarmament.
He did refer to the Treaty on the Prohibition of Nuclear Weapons, which Japan has refused to participate in, but said it is the country's duty to continue working toward the abolishment of nuclear weapons.
In a video message, UN Secretary General Antonio Guterres, who had to cancel his initial plan to be part of the event due to the pandemic, said: "The only way to totally eliminate nuclear risk is to totally eliminate nuclear weapons."
However, about 80 countries and the European Union sent representatives to the event, roughly the same number as in recent years.
A uranium-core atomic bomb named "Little Boy" was dropped by a US bomber exploded above Hiroshima 75 years ago, killing an estimated 140,000 people by the end of 1945, The Japn Times reported.
A second atomic bomb was dropped on Nagasaki on August 9. Japan surrendered six days later, marking the end of World War II.
The combined number of survivors from the two atomic bombings stood at 136,682 as of March, down about 9,200 from a year earlier, the Health, Labor and Welfare Ministry said, with an average age of 83.31.
The city government of Hiroshima has enrolled a further 4,943 people in the past year on the list of people who died from the atomic bombing, bringing the death toll to 324,129.