HIROSHIMA, Japan - Hiroshima's mayor urged global leaders on Thursday to back President Barack Obama's call to abolish nuclear weapons as Japan marked the 64th anniversary of the world's first atomic bomb attack.

In April, Obama said that the United States - the only nation that has deployed atomic bombs in combat - has a "moral responsibility" to act and declared his goal to rid the world of the weapons.

At a solemn ceremony to commemorate the victims of the Aug. 6, 1945, attack, Hiroshima's Mayor Tadatoshi Akiba welcomed that commitment.

"We refer to ourselves, the great global majority, as the 'Obamajority,' and we call on the rest of the world to join forces with us to eliminate all nuclear weapons by 2020," Akiba told a crowd of thousands. The bombed-out dome of the building preserved as the Hiroshima Peace Memorial loomed in the background, and hundreds of white doves were released into the air as he finished speaking.

Hiroshima was instantly flattened and an estimated 140,000 people were killed or died within months when the American B-29 bomber Enola Gay dropped its deadly payload in the waning days of World War II. Hundreds of thousands more have died of related injuries or sickness in the decades since.

Three days after that attack on Hiroshima, the U.S. dropped a plutonium bomb on the city of Nagasaki, killing about 80,000 people. Japan surrendered on Aug. 15, ending World War II.

Japanese Prime Minister Taro Aso also spoke at Thursday's ceremony, saying he hoped the world would follow Tokyo's efforts to limit nuclear proliferation.

The anniversary passed during a period of heightened tensions in the region, just months after North Korea conducted an underground nuclear test blast in May.

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Associated Press writer Jay Alabaster contributed to this report.