UN head Antonio Guterres attends a session. File photo
At an annual memorial, Guterres warned of the risk posed by crises in Ukraine, the Middle East and the Korean peninsula as he described the horrors endured by the Japanese city.
"Tens of thousands of people were killed in this city in the blink of an eye. Women, children and men were incinerated in a hellish fire," he said.
Survivors were "cursed with a radioactive legacy" of cancer and other health problems.
"We must ask: What have we learned from the mushroom cloud that swelled above this city?"
Around 140,000 people died when Hiroshima was bombed by the United States on August 6, 1945, a toll that includes those who perished after the blast from radiation exposure.
The Hiroshima bombing.
Today, "crises with grave nuclear undertones are spreading fast," Guterres said, repeating warnings he made this week at a nuclear Non-Proliferation Treaty conference in New York.
"Humanity is playing with a loaded gun."
Before dawn, survivors and their relatives began to gather at Hiroshima's Peace Memorial Park to pay tribute to the victims.
A silent prayer was held at 8.15 am, the moment the bomb was dropped.
The Russian ambassador was not invited to the ceremony but visited Hiroshima on Thursday to lay flowers at the memorial site.
President Vladimir Putin attends a meeting. File photo
Since Russia invaded Ukraine in February, President Vladimir Putin has made thinly veiled threats hinting at a willingness to deploy tactical nuclear weapons.
In a speech on Saturday, Hiroshima Mayor Kazumi Matsui cited Leo Tolstoy, the Russian author of "War and Peace", saying: "Never build your happiness on the misfortune of others, for only in their happiness can you find your own."
Three days after the Hiroshima bombing, Washington dropped a second atomic bomb on the Japanese port city of Nagasaki, killing about 74,000 people and leading to the end of World War II.
There are now fewer than 119,000 officially recognised survivors of the two nuclear attacks, according to government statistics from March.
The United States remains the only country ever to have used nuclear weapons in conflict.
But around 13,000 are now held in state arsenals worldwide, Guterres said.
Saturday was the first time Guterres attended the Hiroshima memorial in person as UN chief, with a visit last year cancelled because of the Covid-19 pandemic.
"Any attack to a nuclear plant is a suicidal thing," Guterres told a news conference in Japan, where he attended the Hiroshima Peace Memorial Ceremony on Saturday to commemorate the 77th anniversary of the world's first atomic bombing.
The nuclear plant, which is Europe's biggest, was captured by Russian forces in March shortly after the start of Moscow's invasion of Ukraine. Ukraine has accused Russia of deploying soldiers and storing ammunition and armoured vehicles there.
Captured by Russian troops in March but run by Ukrainian staff, Zaporizhzhia has been a hotspot in a conflict that has settled into a war of attrition fought mainly in Ukraine's east and south six months after Russia launched its invasion.
Sheikh Mohammed on Wednesday met a number of high-ranking military and police officials from the UAE and other nations, who attended the graduation ceremony of the 47th cohort of graduates of the Zayed II Military College.
Dubai Police and Civil Defence officials evacuated the building in recording time.
The two leaders also discussed bilateral cooperation across various domains, affirming their joint keenness for continuous consultation to benefit the mutual interests of their countries and peoples.