Ageing survivors, relatives, government officials and foreign delegates, including US Ambassador to Japan Caroline Kennedy, were to observe a moment of silence at 8:15 am local time (4:45 IST), when the detonation turned the western Japanese city into an inferno.
An American B-29 bomber named Enola Gay dropped an atomic bomb on Hiroshima on August 6, 1945, in one of the final chapters of World War II. It had killed an estimated 140,000 by December that year.
Three days later, the port city of Nagasaki was also bombed, killing an estimated 70,000 people.
Japan surrendered on August 15, 1945, bringing the war to a close.
Historians have long been at odds over whether the twin attacks brought a speedier end to the war by forcing Japan's surrender and preventing many more casualties in a planned land invasion.
The bombed cities have long been spearheading anti-nuclear movements, calling atomic bombs "the absolute evil".
Last week, US media reported the death of Theodore Van Kirk, the last surviving crewman of the Enola Gay, who passed away aged 93.
A funeral was reportedly scheduled for August 5 in his hometown of Northumberland, Pennsylvania, which would coincide with the Hiroshima anniversary in Japan.
Many atomic bomb survivors, known as "hibakusha", oppose both military and civilian use of nuclear power, pointing to the tens of thousands who were killed instantly in the Hiroshima blast and the many more who later died from radiation sickness and cancer.
Anti-nuclear sentiment flared in Japan after an earthquake-sparked tsunami left some 19,000 dead or missing and knocked out cooling systems at the Fukushima Daiichi nuclear plant in 2011.
None of those deaths were directly attributed to the nuclear crisis. But reactor meltdowns spread radiation over a large area and forced thousands to leave their homes in the worst atomic disaster since Chernobyl in 1986.
Despite strong public opposition, Japan's nuclear watchdog last month said that two atomic reactors were safe enough to switch back on.
The decision marked a big step towards restarting the country's nuclear plants which were shut after the disaster, and sparked accusations that the regulator was a puppet of the powerful atomic industry.