The mayor of Hiroshima urged Japan to sign a landmark UN treaty banning nuclear weapons as the city on Tuesday marked 74 years since being targeted in the world's first atomic attack.

Prime Minister Shinzo Abe led commemorations at the Peace Memorial Park in Hiroshima as residents offered silent prayers, lit candles and placed wreathes to remember those killed in the August 6, 1945 bombing.

And mayor Kazumi Matsui used the occasion to push the Abe administration to sign the UN treaty on the prohibition of nuclear weapons (TPNW), approved by more than 120 ­nations, but rejected by the US and other nuclear-armed countries.

"I call on the government of the only country to experience a nuclear weapon in war to accede to the hibakusha's [atomic bomb victims] request that the TPNW be signed and ratified," he said. "I urge Japan's leaders to manifest the pacifism of the Japanese constitution by displaying leadership in taking the next step toward a world free from nuclear weapons."

The mayor urged world leaders to come to the city to see the memorial for themselves.

Japan remains the only country to have experienced atomic attack - against Hiroshima and Nagasaki, days ahead of the country's surrender on August 15, 1945 to end World War II.