Eight years after the 9/11 attacks, President Barack Obama said the nation "will never falter" in its pursuit of al Qaeda and its allies.
Mr. Obama placed a wreath under rainy skies at the Pentagon Friday in memory of those who died there on Sept. 11, 2001, as ceremonies marked the eighth anniversary of the terror attacks that killed in Washington, New York City and in a field in Shanksville, Pa.
"Eight Septembers have come and gone. Nearly 3,000 days have passed; almost one for each of those taken from us," the president said. "But no turning of the season can diminish the pain and the loss of that day. No passage of time and no dark skies can ever dull the meaning of this moment. So on this solemn day, at this sacred hour, once more we pause, once more we pray, as a nation and as a people."
President Obama said the nation came together as one after the terror attacks, "united not only in our grief but in our resolve to stand up for the country we love."
He said that renewal of common purpose is the strongest rebuke against the terrorist attackers.
"Today we honor the dead and speak to the survivors and loved ones whose lives are irrevocably changed on that terrible day eight years ago.
"Because of the great pinnacle of their sacrifice and because of the sacrifice of thousands more since that day we remain a strong and free nation," he said.
"Let us renew our resolve against those who perpetrated this barbaric act and who plot against us still. In defense of our nation, we will never waver. In pursuit of al Qaeda and its extremist allies, we will never falter.
"Let us renew the true spirit of that day: not the human capacity for evil, but the human capacity for good; not the desire to destroy, but the impulse to save and to serve and to build. On this first National Day of Service and Remembrance, we can summon once more that ordinary goodness of America to serve our communities, to strengthen our country and to better our world."
He then met with families of the victims at the Pentagon, where 184 people died.
CBS News correspondent Kimberly Dozier reports that, for many here, coming to work brings constant reminders of September 11 - from the memorial outside where the jetliner hit, in honor of those who died here, to the photos of some who were lost, staring down from the inner hallways, and the quilts and signs still posted, sent here by ordinary Americans to honor that sacrifice.
And every time the wounded from Afghanistan and Iraq visit these hallways, from Walter Reed or Bethesda Naval Hospital, that, says Dozier, is another reminder - the wars launched that day and afterward still continue.
Earlier, the president and Mrs. Obama stood at the South Lawn of the White House with heads bowed, and then hands on their hearts, as chimes rang out and the mournful sound of "Taps" filled the air, reports CBS News White House correspondent Peter Maer.
A new CBS News poll finds Americans are mixed on Mr. Obama's efforts to protect the country - around 25 percent feel the administration's policies have made the country more secure, but nearly as many feel the U.S. is less safe.
Marking his first 9/11 anniversary as president, Mr. Obama was joined by dozens of staffers including his chief of staff, the chef in his white coat, uniformed Secret Service guards and many others.
A steady rain stopped just as the simple and poignant White House ceremony began, Maer said, and as the last notes of "Taps," echoed across the lawn, it started pouring again.
Eight years after the terrorist attacks that killed nearly 3,000 people, many Americans are.
Drawing on the spirit that spurred volunteers to rush to the burning World Trade Center on September 11, 2001, Americans looked for ways to help each other.
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Friday was the first time the anniversary was observed as a national day of service, following an order signed this year by President Barack Obama.
Teresa Mathai, whose husband, Joseph Mathai, died at the World Trade Center eight years ago Friday, planned to grieve at a morning wreath-laying ceremony in Boston and hear his name read out loud. Then she planned to install drywall at a low-income home in south Boston with Habitat for Humanity, one of thousands of volunteer efforts planned since Sept. 11 was declared a national day of service.
"Everyone has a different way of mourning," she said. "Some people keep it absolutely sacred. For me, this is something that gives us solace."
"From this day forward, we will safeguard the memories of those who died by rekindling the spirit of service that lit our city with hope and helped keep us strong," said New York Mayor Michael Bloomberg said at a ceremony in lower Manhattan.
Thousands were expected at now-familiar ceremonies in New York, at the Pentagon and at the crash site of United Airlines Flight 93 in Shanksville, Pennsylvania.
In New York, a smaller-than-usual crowd of just hundreds gathered under a veil of cold, steady rain to observe the anniversary of the worst attacks in the nation's history.
Families used rain jackets and umbrellas to fend off the downpour as bells tolled at nearby Trinity Church.
"It doesn't matter what kind of weather there is. I would be here either way. It's a way to come together and find a common place," said Elaine Dejesus of Clifton, New Jersey. She carried a framed photo of Nereida Dejesus, who was her sister and best friend.
Relatives and friends of victims were allowed on Friday to visit the plaza for the Sept. 11 memorial that is under construction. It is expected to be partially complete and open for the 10th anniversary.
New York City Police bagpipers and drummers lead the World Trade Center flag to the Ground Zero stage, where the names of the Trade Center victims were read, with pauses for moments of silence at the minutes the jetliners crashed into the towers.
People involved in volunteer work across the nation joined relatives of victims to read the names of those lost.
One reader represented a group called New York Says Thank You, which sends volunteers from New York City each year on the attack's anniversary to help rebuild communities around the country affected by disasters as a way to send thanks for the help that came to New York City after Sept. 11.
Other readers were from well-known service organizations including the American Red Cross and the United Way.
Some victims' relatives said they feared the emphasis on volunteerism would overshadow a somber day of remembrance for the nearly 3,000 people killed when hijacked jetliners crashed into the Trade Center, the Pentagon and a field in Shanksville, Pa.
Biden spoke during a pause in the reading of the names, telling the several hundred victims' relatives gathered that "there's a special fraternity for those of us who've lost spouses and children." Biden's daughter and first wife died in a 1972 automobile accident.
Before he spoke, Biden and his wife, Jill, joined some family members to lay flowers in one of the reflecting pools at the memorial.
The attacks killed 40 people in Pennsylvania, 184 at the Pentagon and 2,752 in New York.
This year, one new name will be read - a victim added to New York's death toll in January. The medical examiner's office ruled that Leon Heyward, who died last year of lymphoma and lung disease, was a homicide victim because he was caught in the toxic dust cloud just after the towers collapsed.
It's the second time the city has added to the victims' list someone who died long after Sept. 11, ruling that exposure to toxic dust caused lung disease.
Bells tolled throughout New York City for the first of four moments of silence - for when jetliners crashed into each tower and for when each tower collapsed.
Dejesus, wiping tears off her cheeks, said the anniversaries don't get any easier.
"For me, it's just the same as it was the first day," she said. "There are days I just sit there and cry. But I also remember the fun times and what she would want us to do."
At dusk, a tribute in light will shine two beams into the night sky, reports CBS News correspondent Chris Wragge.
Tommy Franks were among hundreds of people gathered under gray skies in a field in Shanksville, Pennsylvania, the site where a United Airlines jetliner fell to Earth.
The names of the 40 passengers and crew were read at 10:03 a.m., the time the plane crashed.
Flight 93 was traveling from Newark, N.J., to San Francisco when hijackers took it over and directed it towards Washington, with the likely goal of crashing it into the White House or Capitol building.
Powell told victims' families that while their losses are painful, thousands of other lives were saved because passengers and crew fought back against the hijackers.
Powell said the 40 victims of Flight 93 are worthy successors to the heroes of U.S. battles dating back to the Revolutionary War.
"As much as we may try we can never truly grasp the emotions that must've gone through the hearts and minds of the passengers and crew as they realized the mortal danger they were in. The choice before them was clear and daunting. If they had any chance of survival they had to attack - not only to save themselves but to save hundreds if not thousands of their fellow citizens and to show the terrorists that they would never defeat us. They refused to let it just happen, they acted. Forty men and women, mostly strangers to each other, but representing the very best and most wonderful diversity that is America.
"The terrorists forced them to the rear of the plane thinking that would keep them under control and out of the way. But they misunderstood Americans. We fight back, we will never be cowed."
"They seized the moment and lost their lives in so doing," Powell said. "But not before forcing the monsters to abort the mission and crash in this field saving so many of their fellow citizens.
"And it is now our moment to act by marching steadfastly toward the goal of completing a memorial to these heroes of 9/11."
Jose Melendez-Perez, a customs agent credited with refusing U.S. entry to a man whom officials believe was supposed to be the fifth hijacker aboard the flight, was going to the site for the first time.
Former President George W. Bush had no public appearances planned Friday, and a spokesman said he would be working in his office during the morning. In a brief statement, he said he and his wife, Laura, were thinking of the victims and their families.
"We honor those who volunteer to keep us safe and extend the reach of freedom - including members of the armed forces, law enforcement officers, and intelligence and homeland security professionals," the statement said. "Their courage, service, and sacrifice is a fitting tribute to all those who gave their lives on Sept. 11, 2001. On this day, let us renew our determination to prevent evil from returning to our shores."