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.
Amid fierce battle between Russia and Ukraine, a video of a Ukrainian removing a anti-tank mine with bare hands with a cigarette in is mouth has gone viral.
Governor Dmytro Zhyvytskyy posted photographs of the charred shell of a four-story building and rescuers searching rubble. In a later Facebook post, he said many Russian soldiers and some local residents also were killed during the fighting on Sunday.
Kathy Lueders, who heads the agency's human spaceflight program, told reporters on a call that operations on the research platform were proceeding "nominally" and "we're not getting any indications at a working level that our counterparts are not committed."
While addressing the ceremony shown live on Pakistani TV channels, General Bajwa expressed his gratitude for representing the Queen, terming it a "unique honour and great privilege.”
Trump's lawyers announced that they would not oppose obtaining the search warrant for his Florida residence unsealed from the public, the Justice Department said in a court filing, indicating the opening could take place shortly.
The Integrated Transport Centre (ITC), affiliated to the Department of Municipalities and Transport - Abu Dhabi, announced the opening of several roads with new signalized intersections on Al Reem Island, starting Saturday, August 13, to serve the public and enhance traffic flow and safety on the roads.