HIROSHIMA, Japan  With the mournful gong of a Buddhist temple bell and the release of a flock of doves, a crowd of 55,000 on Friday solemnly marked the moment 65 years ago when the world’s first atomic attack incinerated this city under a towering mushroom cloud.

For first time, a representative of the United States, Ambassador John V. Roos, participated in the annual ceremony, raising hopes here of a visit soon by a more prominent guest, President Obama, who is scheduled to be in Japan in November.

Mr. Obama has become a popular figure here since delivering a speech last year in Prague calling for the elimination of nuclear weapons. The mayor and other residents of Hiroshima have repeatedly invited him to come to their city, which  along with Nagasaki  is one of the world’s most recognized symbols of the horrors of nuclear war.

Until Friday, American officials had always skipped the annual ceremony, fearing their presence would renew the debate over whether the United States should apologize for the World War II bombings, which together killed more than 200,000 people in explosions so intense that many victims were vaporized, leaving only ghostly shadows on walls, while others died in agony from burns and radiation sickness.