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 the attack on Hiroshima, the U.S. dropped another atomic 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.

Hiroshima's mayor urged global leaders on Thursday to back President 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.

About 50,000 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.

The American occupation forces imposed strict censorship on Japan, prohibiting anything "that might, directly or by inference, disturb public tranquility" and used it to prohibit all pictures of the bombed cities. The pictures remained classified ‘top secret’ for many years. Some of the images have been published later by different means, but it’s not usual to see them all together. This is the horror they didn’t want us to see, and that we must NEVER forget.

... but whats the significance of the 64 year anniversary

Talk about the remembering for the loss of life but don't look to gain much support for your effort with a post like that. War sucks but its reality.

The events that took place in a remote area of New Mexico during the predawn hours of July 16, 1945 forever changed the world. In the early morning darkness the incredible destructive powers of the atom were first unleashed and what had been merely theoretical became reality.

In the early morning hours of August 6, 1945, a B-29 bomber named Enola Gay took off from the island of Tinian and headed north by northwest toward Japan.

Japan was ready to call it quits anyway. More than 60 of its cities had been destroyed by conventional bombing, the home islands were being blockaded by the American Navy, and the Soviet Union entered the war by attacking Japanese troops in Manchuria.

A demonstration explosion over Tokyo harbor would have convinced Japan's leaders to quit without killing many people.

Even if Hiroshima was necessary, the U.S. did not give enough time for word to filter out of its devastation before bombing Nagasaki.

The bomb was used partly to justify the $2 billion spent on its development.

The two cities were of limited military value. Civilians outnumbered troops in Hiroshima five or six to one.

Japanese lives were sacrificed simply for power politics between the U.S. and the Soviet Union.

Conventional firebombing would have caused as much significant damage without making the U.S. the first nation to use nuclear weapons.

Don't forget the "Unconditional Surrender" nonsense that was our policy at the time. Japan refused to surrender unless they could have guarantees that the Emperor would not be punished for war crimes. America said "No. Unconditional surrender is all we'll accept". ....But guess what - We decided not to prosecute the Emperor afterall.

The war effort in Korea and China included the mobilization of labor, in which the workers could be moved to various parts of the Japanese Empire, all to insure production for the war effort. Koreans worked in factories and mines in Manchuria, northern Korea and Japan. About 4,000,000 Koreans and Chinese (6) were displaced from their homes and shipped to these factories so that they would be productive in Japan's war efforts. The war mobilization scattered Koreans and Chinese, as manpower, all over East Asia, in an effort to maximize production for the mother country.

On the evening of July 26, 1945 in San Francisco (which in Tokyo was the morning of July 27) a message from the Allies now commonly known as the Potsdam Proclamation was broadcast in Japanese. The broadcast was relayed to the Japanese government on the morning of the 27th (Pacific War Research Society, The Day Man Lost, pg. 211-212).

The proclamation demanded "the unconditional surrender of all the Japanese armed forces" (U.S. Dept. of State, Foreign Relations of the U.S., The Conference of Berlin (Potsdam), vol. 2, pg. 1474-1476). It made no mention of Japan's central surrender consideration: the retention of the Emperor's position (Butow, pg. 138-139). What made this crucial was that the Japanese believed their Emperor to be a god, the heart of the Japanese people and culture (Pacific War Research Society, Japan's Longest Day, pg. 20). The absence of any assurance regarding the Emperor's fate became Japan's chief objection to the Potsdam Proclamation (Pacific War Research Society, The Day Man Lost, pg. 212-214). In addition, the proclamation made statements that, to the Japanese, could appear threatening to the Emperor: "There must be eliminated for all time the authority and influence of those who have deceived and misled the people of Japan into embarking on world conquest" and "stern justice shall be meted out to all war criminals" (U.S. Dept. of State, Potsdam 2, pg. 1474-1476).