TENS of thousands of people have marked the anniversary of the atomic bombing of Hiroshima, as a rising tide of anti-nuclear sentiment swells in post-Fukushima Japan.

Ageing survivors, relatives, government officials and foreign delegates attended the annual ceremony at Hiroshima Peace Memorial Park commemorating the US bombing of the western Japanese city nearly seven decades ago.

An American B-29 bomber named Enola Gay dropped an atomic bomb on August 6, 1945, turning the city into a nuclear inferno and killing an estimated 140,000 in the final chapter of World War II.

At 8.15am, the time of detonation, the toll of a bell set off a moment of silence. Pedestrians came to a standstill and bowed slightly, joining their hands in prayer under scorching sunshine.

"On this day, in this city, let me proclaim again: there must never be another nuclear attack - never," said Angela Kane, UN high representative for disarmament affairs, reading a message from Secretary General Ban Ki-moon.

"Such weapons have no legitimate place in our world. Their elimination is both morally right and a practical necessity in protecting humanity."

Some 50,000 people attended the official ceremony, while thousands of others joined demonstrations, marches, forums, and concerts held across the city, a long-time focal point for the global movement against nuclear weapons.

Among the attendees was Clifton Truman Daniel, 55, grandson of former US president Harry Truman, who authorised the bombing of Hiroshima and the port city of Nagasaki three days later.

The Allied powers have long argued that the bombings brought a quick end to the war by speeding up Japan's surrender, preventing millions more casualties from a land invasion planned for later in the year.

Daniel is the first Truman relative to attend the anniversary event in Japan.

Around 700 people including atomic bomb survivors and evacuees from the Fukushima area also staged an anti-nuclear rally, the latest in a series of protests triggered by last year's atomic crisis.

An earthquake-sparked tsunami left some 19,000 dead or missing and knocked out cooling systems at the Fukushima Daiichi plant, causing meltdowns that spread radiation over a large area and forced thousands to leave their homes.

Usually sedate Japan has seen a string of anti-nuclear protests since Prime Minister Yoshihiko Noda in June ordered the restart of two reactors.

Many atomic bomb survivors, known as "hibakusha", oppose both military and civil 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 cancers linked to the attack.

Demonstrators marched around the headquarters of Chugoku Electric Power, a regional utility which has reactors of its own, chanting: "Noda should quit. We oppose nuclear power."

Weekly demonstrations outside the prime minister's official residence have drawn thousands, while a rally in west Tokyo last month saw a crowd that organisers claimed swelled to 170,000.

There are fears it could be decades before the area around Fukushima is deemed safe for human habitation.