Weeping relatives of the victims streamed into a newly opened memorial and placed pictures and flowers beside names etched in bronze. Obama and his predecessor, George W. Bush, bowed their heads and touched the inscriptions.
Obama, standing behind bulletproof glass and before the white oak trees of the memorial, read the Bible passage after a moment of silence at 8:46 a.m., when the first jetliner slammed into the north tower 10 years ago.
The president, quoting Psalm 46, invoked the presence of God as an inspiration to endure. "Therefore, we will not fear, even though the earth be removed, and though the mountains be carried into the midst of the sea."
The New York ceremony was the centerpiece of a day of remembrance across the country. It was a chance to reflect on a decade that changed American life, including two wars and the overhaul of everyday security at airports and in big cities.
In a ceremony at the Pentagon, Defense Secretary Leon Panetta observed a moment of silence at 9:37 a.m., marking the time a jet struck the center of the nation's military. He paid tribute to 6,200 members of the U.S. military who have died in the Iraq and Afghan wars.
In Shanksville, Pa., a choir sang at the Flight 93 National Memorial, and a crowd of 5,000 listened to a reading of the names of 40 passengers and crew killed aboard the plane a decade ago.
In New York, family members began reading the names of 2,983 victims -- 2,977 killed in New York, Washington and Pennsylvania on Sept. 11, 2001, and six killed in the first terror attack on the trade center, a truck bomb in 1993.
"You will always be my hero," Patricia Smith, 12, said of her mother.
Nicholas Gorki remembered his father, "who I never met because I was in my mother's belly. I love you, Father. You gave me the gift of life, and I wish you could be here to enjoy it with me."
Peter Negron, 21, whose father worked on the 88th floor of the north tower, said that in the decade since the attack, he had tried to teach his younger brother lessons he had learned from their father.
"I decided to become a forensic scientist," Negron said. "I hope that I can make my father proud of the young men my brother and I have become. I miss you so much, Dad."
Bush quoted a letter from President Abraham Lincoln to the mother of soldiers who died in the Civil War.
"I pray that our heavenly father may assuage the anguish of your bereavement," Bush said.
Obama and Bush were joined by their wives as they walked up to one of the two reflecting pools built over the towers' footprints, part of a Sept. 11 memorial that was opened for relatives of the victims.
Some family members held children on their backs who were not yet born when the towers were attacked.
Mayor Michael Bloomberg, opening the ceremony of remembrance, said: "Although we can never un-see what happened here, we can also see that children who lost their parents have grown into young adults. ... Good works have taken root in public service."
As the sun rose, an American flag fluttered over six stories of the rising 1 World Trade Center. The sky was clear blue with scattered white clouds and a light breeze, not unlike the Tuesday morning 10 years ago.
The site looked utterly different than it had for any other Sept. 11 anniversary: Along with the names in bronze, there were two manmade waterfalls directly on the footprints of the towers, surrounded by dozens of white oak trees.
Elijah Portillo, 17, whose father was killed in the attack, said he had never wanted to attend the anniversary because he thought he would feel angry. But this time was different, he said.
"Time to be a big boy," Elijah said. "Time to not let things hold you back. Time to just step out into the world and see how things are."
The anniversary arrived with security officials in New York and Washington on alert. Ahead of the anniversary, the federal government had warned local authorities of a tip about a possible car-bomb plot.
Remembrances around the nation and world marked a decade of longing for loved ones lost in the attack.
The anniversary revived memories of a September morning when terrorists crashed hijacked planes into the World Trade Center and the Pentagon, and a fourth plane crashed into a field in rural western Pennsylvania. Of heroism and Samaritans and unthinkable fear. And of nearly 3,000 killed at the hands of a global terror network led by Osama bin Laden, himself now dead.
In a taped interview, the president told NBC News that the United States "came through this thing in a way that was consistent with our character."
"We've made mistakes. Some things haven't happened as quickly as they needed to," he said. "But overall, we took the fight to al-Qaida, we preserved our values, we preserved our character."
People across America planned to gather to pray at cathedrals in their greatest cities and to lay roses before fire stations in their smallest towns. Around the world, many others planned to do something similar.
On Saturday in rural western Pennsylvania, more than 4,000 people began to tell the story again.
At the dedication of the Flight 93 National Memorial near the town of Shanksville, Bush and former President Bill Clinton and Vice President Joe Biden joined the families of the 40 passengers and crew aboard the jet who fought back against their hijackers.
"The moment America's democracy was under attack our citizens defied their captors by holding a vote," Bush said. Their choice cost them their lives.
The passengers and crew gave "the entire country an incalculable gift: They saved the Capitol from attack," an untold amount of lives and denied al-Qaida the symbolic victory of "smashing the center of American government," Clinton said.
They were "ordinary people given no time at all to decide and they did the right thing," he said.
"And 2,500 years from now, I hope and pray to God that people will still remember this."
The Pennsylvania memorial park is years from completion. But the dedication and a service to mark the 10th anniversary of the attacks are critical milestones, said Sally Ware, one of the volunteer "ambassadors" who has worked as a guide at the site since the disaster.
Ware, whose home was rocked when the jet crashed two miles away, recalled how hundreds of people flocked to the site in the days afterward to leave their own mementos and memorials. She began volunteering after finding one along the roadside - a red rose placed atop a flight attendant's uniform.
"It really bothered me. I thought someone has to take care of this," said Ware, whose daughter is a flight attendant.
Now, a decade later, she said the memorial may do little to ease the grief of the families of those who died in the crash.
But the weekend's ceremonies recall a story with far broader reach. The ceremonies honor those who "fought the first battle against terrorism -- and they won," Ware said. "It's something I don't want to miss. It's become a part of my life."
As the anniversary arrived around the world, people paid tribute in formal ceremonies and quiet moments.
In Japan, they gathered Sunday to lay flowers before a glass case containing a small section of trade center steel, and remembered 23 employees of Fuji Bank who never made it out of the towers.
A village in the Philippines offered roses, balloons and prayers for an American victim whose widower built 50 brightly colored homes there, fulfilling his late wife's wish to help the Filipino poor.
In Malaysia, Pathmawathy Navaratnam woke up and, as she has done every morning for 10 years, wished "good morning" to her son, a 23-year-old financial analyst who was killed in New York.
"He is my sunshine. He has lived life to the fullest, but I can't accept that he is not here anymore," said Navaratnam. "I am still living, but I am dead inside."
In a reminder of the war that started in the wake of the attacks, 77 American soldiers were wounded when a Taliban suicide bomber detonated a truck bomb outside the gates of a U.S. base in eastern Afghanistan. Two Afghans were also killed.
On Sunday, the focus turned to ceremonies at the Pentagon, just outside Washington, D.C., and in lower Manhattan. Obama planned to attend events at the sites and was to speak at a Sunday evening service at the Kennedy Center.
Among the names being read in New York were those of 37 of Lt. Patrick Lim's fellow officers from the police department of the Port Authority of New York and New Jersey.
Lim, assigned to patrol the trade center with an explosives detection dog, rushed in to the north tower after it was hit to help evacuate workers. He and a few others survived despite still being inside a fifth-floor stairwell when the building fell.
In the years since, Lim said he has wrestled with survivor's guilt. He took shelter in selective memory, visualizing the ground covered with women's shoes amid the destruction. "That's how I got through that because what was attached to the shoes was a lot worse," Lim said.
The 10th anniversary has forced Lim to revisit an experience he's worried too many people have pushed from their minds. But the approach of Sunday's ceremonies has convinced him of the value of revisiting Sept. 11, both for himself and others.
When it happened, talking about the events of that day "wasn't easy for me. This was very difficult. But it became ... a catharsis," he said. "What I want is for people to remember what happened."
The hundreds of ceremonies across the country and around the globe included memorial Mass at St. Patrick's Cathedral in New York and a ceremony featuring nine-stories-tall replicas of the twin towers on a plaza in Paris.
But some of the most powerful ceremonies will likely be the smallest and most personal.
In Newtown, Conn., retired American Stock Exchange floor broker Howard Lasher planned a ceremony Sunday morning under the canopy of six maple trees standing alongside his gravel driveway; their trunks are painted to resemble an American flag.
Lasher commissioned the painting as a tribute to nine colleagues and the son of another who died inside the trade center.
"I wanted something that would reach out to people, that people would not forget," Lasher said.
And in tiny Brown City, Mich. -- with no direct connection to the attacks- - firefighters plan to lay 343 roses on a 15,000-pound steel beam salvaged from the World Trade Center, in honor of their New York City brethren who perished. It has already become a local shrine, Chief Jim Groat said.
A few days ago, a couple from St. Joseph, Mich. who happened to be driving through, pulled into the fire station lot when they spotted a sign for the memorial. The woman explained to Groat that she was an American Airlines flight attendant on Sept. 11.
Then she turned to face the steel beam from the trade center and cried. "She said she was just honored that somebody still cares," Groat recalled.
The chief observed silently, before offering an invitation.
"Will I see you here on Sept. 11?" he asked.
"I'll be here," she answered.
Associated Press writer Joe Mandak in Shanksville, Pa., contributed to this report.