Hiroshima's mayor urged global leaders on Thursday to back US 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 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 said. 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. "Together, we can abolish nuclear weapons. Yes, we can," he said. About 50,000 people attended the ceremony, including officials and visitors from countries around the world, though the United States did not have an official representative at the ceremony. 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. Three days after that attack on Hiroshima, the US dropped a plutonium bomb on the city of Nagasaki, killing about 80,000 people. Japan surrendered on Aug. 15, ending World War II. A total of about 260,000 victims of the attack are officially recognized by the government, including those that have died of related injuries or sickness in the decades since. 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. "Japan will continue to uphold its three non-nuclear principles and lead the international community toward the abolishment of nuclear weapons and lasting peace," he said. The three principles state that Japan will not make, own or harbor nuclear weapons. Later in the day, Aso signed an agreement with a group of atomic bomb survivors who had been seeking recognition and expanded health benefits from the government. The government gives those people certified as survivors free medical service. But it also provides a monthly allowance of 137,000 yen ($1,440) to those determined by the health ministry to be suffering from illnesses caused by the attacks. Thursday's deal will give that allowance to an additional 306 people who had been fighting in court for the past six years to have their illnesses recognized after the government rejected their petitions. Chief Cabinet Secretary Takeo Kawamura said the government wanted to settle the suits once and for all. "The plaintiffs were getting old and their illnesses have worsened," he said. Sunao Tsuboi, a leader of Nihon Hidankyo, a nationwide organization for atomic bomb survivors, welcomed the deal, saying the plaintiffs "finally felt relieved." 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. A similar ceremony will be held in Nagasaki on Sunday.