On the White House South Lawn, President Obama and First Lady Michelle Obama bowed their heads at 8:46 a.m., the moment the first plane hit the World Trade Center’s North Tower.
They later placed a wreath at the Pentagon, where the third plane struck. A flag was draped over the building to mark the day. "Eleven times we have marked another September 11th come and gone," Mr. Obama said to families and military at the Pentagon, where 184 were killed. "Eleven times we have paused in remembrance, in reflection, in unity and in purpose. This is never an easy day."
The anniversary was observed in a more quiet, subdued way; neither Mr. Obama nor Vice President Joe Biden went to New York as they had in past years.
"But no matter how many years pass, no matter how many times we come together on this hallowed ground, know this — that you will never be alone," Mr. Obama said. "Your loved ones will never be forgotten.
"They will endure in the hearts of our nation, because through their sacrifice, they helped us make the America we are today —an America that has emerged even stronger."
At a ceremony in Manhattan, where One World Trade Center is rising in place of the twin towers, families of victims read the names of loved ones killed in the attacks, and traders on the New York Stock Exchange’s floor stood silent.
This time, the faces on the stage were almost all those of the 200 readers listing the dead, one by one, the names of cousins, brothers, mothers, and husbands sounding for almost four hours over the twin reflecting pools that stand where the towers fell 11 years ago.
Other elements of the annual Sept. 11 ceremony at Ground Zero stayed the same: a chorus of children's voices, an honor guard carrying a battered flag salvaged from the trade center, six moments of silence to mark the impact of planes crashing and buildings hitting the ground, three trumpeters closing the day's ceremonies with the haunting sound of "Taps."
Outside the site, however, many places across the country had shrunk their ceremonies or chosen not to hold them.
But at the trade center, the readers reminded the crowd of relatives, friends, police officers, firefighters, and politicians that even if the rest of the country seemed ready to move on, their grief, at least on this day, was still vivid.
"Mark, they say time heals all wounds," said Joanne Hindy, who read the name of her nephew, Mark Hindy. "It's not true, Mark. We love you dearly and miss you more, Mark."
Politicians did attend, although none spoke, under rules Mayor Michael R. Bloomberg imposed this year. The absence of readings, prayers, speeches, or anything but music, bells, and names from the program meant the ceremony was dominated by personal tributes from relatives to their dead. The names varied, but the messages were nearly all the same, repeated by children who never knew their fathers, and wives who lost their husbands, until they became a kind of refrain within the long list of the dead.
When Angelina Jimenez read her mother's name, adding a personal message in Spanish, her wavering voice made her meaning clear, even if not all could understand the words.
It was the same as all the others: We still love you. We still miss you. We have not forgotten.
In Shanksville, Pa., where United Airlines Flight 93 crashed in an open field after being hijacked by four terrorists, Mr. Biden addressed the crowd.
Thousands of miles away, allied military forces marked the anniversary at a short ceremony at NATO's headquarters in Afghanistan with a tribute to more than 3,000 foreign troops killed in the decade-long war. At least 1,987 U.S. troops have died in Afghanistan and 4,475 in Iraq, according to the Pentagon.
In Reno, where he delivered a speech on foreign policy, Republican presidential nominee Mitt Romney shared his memories from Sept. 11, 2001, when he was in Washington as the head of the Winter Olympics, meeting with members of Congress about security preparations for the Salt Lake City Games. He said he watched the second plane hit the World Trade Center on a small television on his desk. Later, he recalled, he left Washington and passed by the Pentagon just after he crossed the Potomac River.
"Cars had stopped where they were and people had gotten out, watching in horror," Mr. Romney said. "I could smell burning fuel and concrete and steel. It was the smell of war, something I never imagined I would smell in America."
Mr. Romney, criticized for failing to mention U.S. troops or the war in Afghanistan in his acceptance speech at the Republican National Convention, said he would help steer a century that began with terrorism, war, and economic calamity onto the path of freedom, peace, and prosperity.
Although national security and foreign policy have taken a back seat to economic issues this election season, some conservatives urged Mr. Romney to take a strong stance in the speech.
According to a recent poll, Mr. Obama has a clear advantage over Mr. Romney on foreign policy and national security. Four in 10 voters say the country is safer since Mr. Obama took office; only one in eight sees the country as less secure. And 51 percent of all voters said they trust Mr. Obama to handle terrorism; 40 percent said they trust Mr. Romney to do so.