Bells tolled as a solemn crowd observed a moment of silence at 8:15 am local time (2315 GMT), when the detonation turned the western Japanese city into an inferno, killing thousands instantly and leaving others to die a slow death with horrible injuries.
Children, elderly survivors and delegates representing 100 countries were in attendance with many placing flowers in front of the cenotaph at Peace Memorial Park in downtown Hiroshima.
Prime Minister Shinzo Abe, US ambassador Caroline Kennedy, and under-secretary for arms control Rose Gottemoeller, the most senior Washington official ever sent to the service, were in attendance.
"As the only country ever attacked by an atomic bomb... we have a mission to create a world without nuclear arms," Abe told the crowd.
"We have been tasked with conveying the inhumanity of nuclear weapons, across generations and borders."
The premier said his country would submit a fresh resolution to abolish nuclear weapons at the UN general assembly later this year.
This year's memorial comes just days ahead of the scheduled restart of a nuclear reactor in southern Japan -- the first one to go back on line after two years of complete hiatus following the tsunami-sparked disaster at Fukushima in 2011.
While Abe's government has pushed to switch reactors back on, public opposition to atomic power remains high after Fukushima, the world's worst nuclear accident since Chernobyl in 1986.
Doves fly over the cenotaph dedicated to the victims of the atomic bombing at the Hiroshima Peace Memorial Park during the ceremony to mark the 70th anniversary of the bombing in Hiroshima, western Japan Thursday, Aug. 6, 2015.
Japanese Prime Minister Shinzo Abe bows in front of the cenotaph dedicated to the victims of the atomic bombing at the Hiroshima Peace Memorial Park during the ceremony to mark the 70th anniversary of the bombing in Hiroshima, western Japan Thursday, Aug. 6, 2015.
People walk past the Atomic Bomb Dome beside Peace Memorial Park at sunset in Hiroshima on August 5, 2015
People visit the Peace Memorial Park to pray for victims of the atomic bombing in Hiroshima on August 5, 2015
Abe, a strident nationalist, has also been criticized at home for his efforts to expand the role of pacifist Japan's Self-Defense Forces, changes that could open the door to putting troops into combat for the first time since the end of the war.
The moves caused a fresh stir as defense minister Gen Nakatani admitted Wednesday that new security laws being debated in parliament could -- in theory -- allow for Japan to transport nuclear weapons to allies. He quickly dismissed that idea as unlikely, however.
- 'Absolute evil' -
On Thursday, Hiroshima's mayor Kazumi Matsui said nuclear weapons were an "absolute evil", as he urged the world to put an end to them forever.
"Now is the time to start taking action," Matsui said.
An American B-29 bomber named Enola Gay dropped a bomb, dubbed "Little Boy", on Hiroshima on August 6, 1945.
Nearly everything around it was incinerated, with the ground level hit by a wall of heat up to 4,000 degrees Celsius -- hot enough to melt steel.
"It was a white, silvery flash," Hiroshima survivor Sunao Tsuboi, 90, told AFP before Thursday's memorial.
"I don't know why I survived and lived this long. The more I think about it... the more painful it becomes to recall."
About 140,000 people are estimated to have been killed in the attack, including those who survived the bombing itself but died in the following days, weeks and months.
On August 9, the port city of Nagasaki was also attacked with an atomic bomb, killing more than 70,000 people.
Japan surrendered days later -- on August 15, 1945 -- bringing the war to a close.
Opinion remains divided over whether the twin attacks were justified.
While some historians say that they prevented many more casualties in a planned land invasion, critics counter that the attacks were not necessary to end the war, arguing that Japan was already heading for imminent defeat.
- 'Did the right thing' -
Dropping the bombs, which were developed under strict secrecy, was hugely popular with war-weary Americans at the time -- and 70 years on, a majority today still think it was the right thing to do.
Fifty-six percent of Americans surveyed by the Pew Research Center in February said using the atomic bomb on Japanese cities was justified, compared to 79 percent of Japanese respondents who said it was not.
Paul Tibbets, who piloted the Enola Gay, said he never had any second thoughts about dropping the bomb, telling a newspaper in an interview in 2002, five years before his death: "I knew we did the right thing".
Washington, which has been a close ally of Tokyo since the war, has never officially apologised for the bombings.
Leaked diplomatic cables from 2009 suggested that the Japanese government had rebuffed the idea of a US apology and a visit to Hiroshima by President Barack Obama.