A peace bell tolled at 8:15 a.m. (2315 GMT on August 5), the time a U.S. warplane dropped the bomb. About 50,000 participants including aging survivors and dignitaries held a moment of silence at a memorial ceremony in the western Japanese city.
Obama this year became the first incumbent U.S. president to visit Hiroshima, and he urged nuclear powers, including his own, to have the courage to escape the logic of fear and pursue a world without nuclear weapons.
"The president's words showed he was touched by the spirit of Hiroshima, which refuses to accept the 'absolute evil'," the mayor, Kazumi Matsui, told the crowd, referring to the weapons.
The United States dropped the bomb on Hiroshima on Aug. 6, 1945, killing thousands of people instantly and about 140,000 by the end of that year.
U.S. forces dropped another atomic bomb on the southern city of Nagasaki on Aug. 9. Japan surrendered six days later.
"I once again urge the leaders of all nations to visit the A-bombed cities," Matsui said as cicadas buzzed away under the mid-summer sun.
"As President Obama confirmed in Hiroshima, such visits will surely etch the reality of the atomic bombings in each heart."
At the ceremony, Prime Minister Shinzo Abe pledged his determination to work toward a world free of nuclear arms.
"We must not have the tragic experience of Hiroshima and Nagasaki 71 years ago repeat itself," Abe said.
"It is the responsibility of those of us who live in the present to keep on working without cease toward that aim."
U.S. Republican presidential nominee Donald Trump has said he would consider letting Japan and South Korea build their own nuclear weapons, rather than rely on the United States for protection against North Korea and China.
But Tomomi Inada, Japan's new defense minister and an Abe ally, said on Wednesday she did not believe Japan should consider possessing nuclear weapons.
North Korea has conducted four nuclear tests, the latest one in January. It has also conducted a string of tests of various missiles this year.
On Wednesday, it launched a ballistic missile that landed in the sea in Japan's exclusive economic zone for the first time.