Around 140,000 people were killed when Hiroshima was bombed by the US on August 6, 1945 –– a toll that includes those who perished after the blast from radiation exposure.
Bells have tolled in Hiroshima as the Japanese city marked the 77th anniversary of the world's first atomic bombing, with officials including the United Nations chief warning of a new arms race following Russia's conflict with Ukraine.
UN head Antonio Guterres on Saturday joined the thousands packed into the Peace Park in the centre of the city to mark the anniversary of the bombing that killed 140,000 people, only the second time a UN head has taken part in the annual ceremony.
"Nuclear weapons are nonsense. They guarantee no safety –– only death and destruction," Guterres said.
"Three-quarters of a century later, we must ask what we've learned from the mushroom cloud that swelled above this city in 1945."
Hiroshima Mayor Kazumi Matsui, whose city this year did not invite the Russian ambassador to the ceremony, was more pointed and critical of Moscow's military actions in Ukraine.
"In invading Ukraine, the Russian leader, elected to protect the lives and property of his people, is using them as instruments of war, stealing the lives and livelihoods of civilians in a different country," Matsui said.
"These errors betray humanity's determination, born of our experiences of war, to achieve a peaceful world free from nuclear weapons. To accept the status quo and abandon the ideal of peace maintained without military force is to threaten the very survival of the human race."
'Nuclear war cannot be won'
At 8:15 am on August 6, 1945, the US B-29 warplane Enola Gay dropped a bomb nicknamed "Little Boy" and obliterated the city with an estimated population of 350,000.
Thousands more died later from wounds and radiation-related illnesses.
On Saturday, as cicadas shrilled in the heavy summer air, the Peace Bell sounded and the crowd, including Prime Minister Fumio Kishida, who is from Hiroshima, observed a moment of silence at the exact time the bomb exploded.
"At the start of this year, the five nuclear-weapon states issued a joint statement: 'Nuclear war cannot be won and must never be fought,'" Matsui added.
"Why do they not attempt to fulfill their promises? Why do some even hint at using nuclear weapons?"
Kishida, who has chosen Hiroshima as the site of next year's Group of Seven summit, called on the world to abandon nuclear weapons.
The Hiroshima catastrophe was followed by the US military's atomic bombing of Nagasaki on August 9, instantly killing more than 75,000 people.
Japan surrendered on August 15, ending World War II and Japan's nearly half-century of aggression in Asia.