Weeping relatives of the victims streamed into a newly opened memorial at the spot where the World Trade Center stood. They placed pictures and flowers beside names etched in bronze, and traced them with pencil and paper. President Barack Obama and his predecessor, George W. Bush, bowed their heads and touched the inscriptions.
Obama, standing behind bulletproof glass and in front of the white oak trees of the memorial, read a 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 an overhaul of everyday security at airports and in big cities.
In a ceremony at the Pentagon, Biden paid tribute to "the 9/11 generation of warriors."
"Never before in our history has America asked so much over such a sustained period of an all-volunteer force," he said. "So I can say without fear of contradiction or being accused of exaggeration, the 9/11 generation ranks among the greatest our nation has ever produced, and it was born -- it was born -- it was born right here on 9/11."
Defense Secretary Leon Panetta observed a moment of silence at 9:37 a.m., marking the time a jet struck the headquarters 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 fourth jetliner hijacked that day a decade ago. Obama and his wife traveled to the Pennsylvania town after their visit to New York and placed a wreath at the memorial.
During the president's visit, members of the crowd chanted, "USA! USA!" One man called out: "Thanks for getting bin Laden!" This is the first anniversary observance since Osama bin Laden was killed by U.S. forces in Pakistan.
The day's events took place under higher security than usual. In New York and Washington especially, authorities were on alert. Ahead of the anniversary, the federal government warned those cities of a tip about a possible car-bomb plot. Police searched trucks in New York, and streets near the trade center were blocked. To walk within blocks of the site, people had to go through checkpoints.
In New York, family members began read aloud 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."
Alex Zangrilli said: "Dad, I wish you were here with me to give me advice, to be on the sidelines when I play sports like all the other dads. ... I wish we had more time together."
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 Abraham Lincoln to a mother who was believed to have lost five sons in battle during in the Civil War: "I pray that our heavenly father may assuage the anguish of your bereavement."
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, 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."
Cellist Yo-Yo Ma played a mournful Bach composition. James Taylor sang the melancholy "Close Your Eyes," and Paul Simon strummed a raw version of "The Sound of Silence."
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 that 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."
In a taped interview, the president told NBC 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 gathered to pray at cathedrals in their greatest cities and to lay roses before fire stations in their smallest towns.
Around the world, people paid tribute in formal ceremonies and quiet moments.
In Japan, they laid 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."
The day's events also included a ceremony featuring nine-story replicas of the twin towers on a plaza in Paris.
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 killed.
Some observed the day as a time to serve. Thousands cleaned parks, renovated community centers and gave blood as they did in the days after the 2001 attacks. Some said they were trying to reclaim goodwill that they said has been lost amid political rancor and economic fear.
"As unfortunate as it was, it seemed like it put us all back into the frame of mind that life wasn't just about me," said Yvette Windham, who joined 200 people to build seven new homes in a Nashville, Tenn., neighborhood.
Associated Press writers Tamara Lush in Nashville, Tenn., and Joe Mandak in Shanksville, Pa., contributed to this report.