Hiroshima (CNN) Barack Obama on Friday became the first sitting U.S. president to visit Hiroshima, where he called for a "world without nuclear weapons" during his remarks at the city's Peace Memorial Park.

Obama said that "71 years ago on a bright, cloudless morning, death fell from the sky and the world was changed."

"A flash of light and a wall of fire destroyed a city, and demonstrated that mankind possessed the means to destroy itself," the President added during his address at the site of the first nuclear bombing.

Obama was not expected to apologize for the U.S. action to hasten the end of World War II and he did not during his 20-minute-long remarks.

"Why do we come to this place, to Hiroshima? We come to ponder the terrible forces unleashed in the not so distant past. We come to mourn the dead ... their souls speak to us and ask us to look inward. To take stock of who we are and what we might become."

In the Hiroshima museum's guest book before his speech, the President wrote that he hoped the world will "find the courage, together, to spread peace, and pursue a world without nuclear weapons.‎"

Photos: Obama makes historic visit to Hiroshima President Barack Obama hugs Shigeaki Mori, an atomic bomb survivor who created the memorial for American WWII POWs killed at Hiroshima, during a ceremony at Hiroshima Peace Memorial Park in Hiroshima, Japan, Friday, May 27. Obama on Friday became the first sitting U.S. president to visit the site of the world's first atomic bomb attack. Hide Caption 1 of 13 Photos: Obama makes historic visit to Hiroshima President Obama, center, accompanied by Japanese Prime Minister Shinzo Abe, left, shakes hands and talks with Sunao Tsuboi, a survivor of the 1945 atomic bombing and chairman of the Hiroshima Prefectural Confederation of A-bomb Sufferers Organization (HPCASO), at Hiroshima Peace Memorial Park in Hiroshima, western Japan. Hide Caption 2 of 13 Photos: Obama makes historic visit to Hiroshima President Obama, right, and Japanese Prime Minister Shinzo Abe shake hands after laying wreaths at the Hiroshima Peace Memorial Park in Hiroshima on May 27. Obama is the first sitting U.S. president to visit Hiroshima. Hide Caption 3 of 13 Photos: Obama makes historic visit to Hiroshima President Obama lays a wreath at Hiroshima Peace Memorial Park Friday. Hide Caption 4 of 13 Photos: Obama makes historic visit to Hiroshima President Obama, right, is greeted Japanese Prime Minister Shinzo Abe upon arriving at the Hiroshima Peace Memorial Park. Hide Caption 5 of 13 Photos: Obama makes historic visit to Hiroshima President Obama walks off Marine One at the landing zone in Hiroshima, western Japan. Hide Caption 6 of 13 Photos: Obama makes historic visit to Hiroshima President Barack Obama and U.S. Ambassador to Japan Caroline Kennedy arrive at the Marine Corps' Air Station Iwakuni in the western Japanese city. Hide Caption 7 of 13 Photos: Obama makes historic visit to Hiroshima U.S. President Barack Obama, foreground center, is greeted by U.S. Marines and their families at Iwakuni air station. Hide Caption 8 of 13 Photos: Obama makes historic visit to Hiroshima President Obama speaks to members of the U.S. and Japanese military at the Marine Corps Air Station Iwakuni in Japan before continuing to Hiroshima. Hide Caption 9 of 13 Photos: Obama makes historic visit to Hiroshima President Obama speaks to U.S. and Japanese troops during his visit to the Marine Corps Air Station. Hide Caption 10 of 13 Photos: Obama makes historic visit to Hiroshima President Obama greets military personnel at the Marine Corps Air Station Iwakuni. Hide Caption 11 of 13 Photos: Obama makes historic visit to Hiroshima Police officers, seen through the cenotaph, patrol at Hiroshima Peace Memorial Park in Hiroshima. Hide Caption 12 of 13 Photos: Obama makes historic visit to Hiroshima A helicopter takes off near the the Atomic Bomb Dome in Hiroshima, Japan. Hide Caption 13 of 13

'Moral revolution'

A somber Obama spoke after laying a wreath on the museum's cenotaph alongside Japanese Prime Minister Shinzo Abe.

During the speech, the park was silent except for a circling helicopter, chirping birds and camera shutters.

Obama said there is a "shared responsibility" to look into the "eye of history" and ask what must be done to prevent another nuclear weapon being used.

He urged that the world make moral progress alongside its remarkable scientific advancements.

"The scientific revolution that led to the splitting of an atom requires a moral revolution as well," Obama said. "That is why we come to this place."

Photos: Remnants of the world's first atomic attack Photos: Remnants of the world's first atomic attack Seventy years after the United States dropped an atomic bomb on Hiroshima, Japan, this toddler's tricycle stands as a bitter reminder of the horrors of nuclear warfare. The story behind it was published as a children's book by Hiroshima survivor Tatsuharu Kodama in 1995. "Shin's Tricycle" is about a 3-year-old boy named Shinichi Tetsutani, who died in the attack. His father buried him with this trike -- his favorite toy. This and other fascinating artifacts have been preserved by the Hiroshima Peace Memorial Museum Hide Caption 1 of 10 Photos: Remnants of the world's first atomic attack Shirley Temple doll – Chieko Suetomo loved this Shirley Temple doll that her father got her when he was in the United States. When he returned to their destroyed house a few days after the attack, she found the doll laying on what was left of the floor. The doll's once-beautiful light-peach clothes were blackened from head to toe, but Chieko continued to treasure the doll after the war. She eventually donated it to the museum. Hide Caption 2 of 10 Photos: Remnants of the world's first atomic attack Lunch box – Shigeru Orimen was a first-year student at Second Hiroshima Prefectural Junior High School. A few days after the bombing, his mother found Orimen's body with this lunch box clutched under his stomach. The bomb had turned his lunch into nothing but charred remains. Hide Caption 3 of 10 Photos: Remnants of the world's first atomic attack A burned shirt – The bomb detonated while Shigezo Kono was at work at a city electric company. Two days later, his older brother found him lying dead, face down beside his desk. He brought home this burned shirt, which had been sewn by Shigezo's wife, Toshi. When she saw it, she knew her husband was dead. Hide Caption 4 of 10 Photos: Remnants of the world's first atomic attack A student's armband – Toshiaki Asahi was a 13-year-old high school student who was working at the time of the attack and wearing this armband. Despite serious burns, he managed to make his way through fires sparked by the bomb. He climbed up a riverbank and escaped to the outskirts of the city. There he was found by an acquaintance and carried home. Three days later, he told family members, "Thank you for all you've done," and died in his mother's lap. Hide Caption 5 of 10 Photos: Remnants of the world's first atomic attack Steel helmet – Teacher Ryo Fukumaru was escorting students to a factory at the time of the attack. He was badly burned over most of his body -- except his head, which was shielded by this helmet. He was able to walk back to his school before he collapsed. Two days later, he was carried on a stretcher back to his family. At first, because of his burns, relatives failed to recognize him. He struggled to heal for more than six months. When he recovered, Fukumaru was left with scars covering most of his body. He was eventually able to return to work. Hide Caption 6 of 10 Photos: Remnants of the world's first atomic attack School bag – On the day of the attack, Mitsuko Kawamura was a 13-year-old high school student. Her sister Yaeko walked through the city searching for her, but never found a body. About a month later, Yaeko found her sister's school bag near the place where Mitsuko was working that day. Hide Caption 7 of 10 Photos: Remnants of the world's first atomic attack Suitcase – When the bomb detonated, Tadayori Kihara was riding his bicycle onto a bridge with this suitcase mounted behind his seat. The blast threw him down on the bridge walkway. His back and arms were so badly burned that most of the skin peeled off. The suitcase bears burns from the bomb's intense heat. Kihara survived and lived 22 more years, treasuring this suitcase before it was donated to the museum. Hide Caption 8 of 10 Photos: Remnants of the world's first atomic attack Wallet – Immediately after the bombing, Tsukushi Nishimura went missing from his workplace. About two weeks later, his remains and his wallet were delivered to his home. Hide Caption 9 of 10 Photos: Remnants of the world's first atomic attack Melted coins – A week after the attack, Kinzo Imura found this clump of melted coins in the burned ruins of a relative's house. This artifact was passed to Imura's nephew, Kazuhiko Ninomiya, who preserved them until donating them to the museum. Hide Caption 10 of 10

The President called for an end to senseless wars, even as he reflected that violence has existed throughout human history.

"The world war that reached its brutal end in Hiroshima and Nagasaki was fought among the wealthiest and most powerful of nations," he said.

"Those civilizations have given the world great cities, magnificent art, thinkers that advanced ideas of harmony and truth, and yet the war grew out of the same base instinct for domination. Conflicts can cause conflicts among the simplest of tribes."

Obama closed his remarks by saying, "The world was forever changed here but today the children of this city will go through their day in peace. What a precious thing that is. It is worth protecting."

Survivors speak

Obama's visit to the bomb site was at least six years in the making inside the White House, and follows a visit by Secretary of State John Kerry last month.

The President's American delegation at the site Friday included National Security Advisor Susan Rice, U.S. Ambassador to Japan Caroline Kennedy, advisor and speechwriter Ben Rhodes, and others.

Over 100,000 Japanese men, women and children, thousands of Koreans, and a dozen American prisoners of war died in the bombing.

Following his speech, the President met with survivors of the bombing. He shared an emotional embrace with Shigeaki Mori, a 79-year-old survivor who worked for four decades to gain official recognition of the 12 Americans killed in the bombing.

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Before the event, another survivor, Sunao Tsuboi, said he "never imagined (the President) would come while I am alive."

"We do not need apologies," Tsuboi added.

"I hope that he will present in Hiroshima what is good for the happiness of humankind. I would like to join hands with each other through the power of reason and beyond hatred."