Updated at 3:22 p.m. ET
(CBS News) President Obama on Tuesday marked the 11th anniversary of the 9/11 terrorist attacks at a memorial service at the Pentagon, noting the that the "true legacy" of that day will be "a safer world, a stronger nation and a people more united than ever before."
"This anniversary gives us faith that even the darkest night gives way to a brighter dawn," the president said. "No single event can ever destroy who we are. No act of terrorism can ever change what we stand for."
Mr. Obama recalled the morning of Sept. 11, 2001 as "a day that began like so many others" but soon left the nation shaken to its core.
"Eleven times we have paused in remembrance and reflection in unity and in purpose," he said. "This is never an easy day, but it is especially difficult for all of you, the families of nearly 3,000 innocents who lost their lives."
The president said that because of their sacrifice, the U.S. has dealt a "crippling blow" to al Qaeda, "Osama bin Laden will never threaten us again," and "our nation is safer and our people are resilient."
Before attending the Pentagon memorial service, Mr. Obama and First Lady Michelle Obama observed a moment of silence at the White House.
After leaving the Pentagon, Mr. and Mrs. Obama made an unscheduled stop at Arlington National Cemetery, where the president visited the graves in Section 60, one of the sections for those killed in Afghanistan and Iraq. The president and first lady walked among the chalk-white markers and put "challenge coins" - medallions bearing insignia passed out by commanders as motivation or to honor achievement - on several of the graves.
Vice President Joe Biden attended the Flight 93 National Memorial in Shanksville, Pennsylvania, where one of the airliners hijacked on 9/11 crashed. "It's a genuine honor to be back here today but like all of the families, we wish we weren't here," Biden said. "It's a bittersweet moment for the entire nation, for all of the country."
Republican presidential candidate Mitt Romney, meanwhile, marked the anniversary in an address to the National Guard Association in Reno, Nevada.
"We remember with heavy hearts the tragic loss of life, and we express thankfulness for the men and women who responded to that tragedy. We honor them, and we honor those who secure our safety even to this day," Romney said.
The Republican candidate added that he would, before an audience like the National Guard, normally discuss the differences between his military plans and Mr. Obama's plans. However, he said, "There is a time and a place for that, but this day is not it."
Romney did note that the U.S. aims to hand over security operations in Afghanistan to Afghan forces by the end of 2014. But the return of troops, he said, "cannot and must not be used as an excuse to hollow out our military through devastating defense budget cuts."
Before boarding his flight to Reno this morning, Romney greeted and paid his respects to the local first responders on the tarmac at the Chicago O'Hare International Airport. The first responders held a moment of silence on the tarmac at 7:46 a.m. CT to mark the anniversary of the attack.
This morning, Romney released a written statement: "Eleven years ago, evil descended upon our country, taking thousands of lives in an unspeakable attack against innocents. America will never forget those who perished. America will never stop caring for the loved ones they left behind. And America shall remain ever vigilant against those who would do us harm. Today we again extend our most profound gratitude to our brave troops who have gone into battle, some never to return, so that we may live in peace. On this most somber day, those who would attack us should know that we are united, one nation under God, in our determination to stop them and to stand tall for peace and freedom at home and across the world."
Romney's running mate Rep. Paul Ryan marked the day by stopping at a fire house in Oak Creek, Pennsylvania for lunch.
After shaking hands with many of the personnel gathered, Ryan took a seat near the center of a table in the fire house and thanked the first responders.
"This is a day where we as Americans need to think and remember the people who lost their lives and be thankful for those of you who put your lives on the line for us every day," he said. "So we are here simply to bring notoriety and a gift of thanks for what you do for us on a daily basis."
After his remarks, Ryan served himself and several others pieces of lasagna.