NEW YORK -- Victims' families and others gathered Tuesday at Ground Zero, the Pentagon and near Shanksville, Pa., to remember the terrorist attacks that devastated the nation 11 years ago.
In New York City, remembrances marked one of the city's most tragic days.
Photos: Remembering 9/11
"It is extremely important that people never forget what happened on Sept. 11," said New York City Fire Commissioner Salvatore Cassano, who is attending many events to commemorate those who lost their lives that day.
Security was tight as police officers in crisp, blue uniforms stood among men in suits with badges attached to their belt buckles. Police cars with flashing lights parked just off the West Side Highway near the World Trade Center site. A giant flag on the Freedom Tower unfurled early as cops stood nearby watching.
Family members read the names of their loved ones who died in the attacks and held pictures as waterfalls from the two memorials babbled in the background. One man held high a sign that had the name Danielle Kousoulis on it with pictures of the young woman and the words: "We miss you" and "We love you."
"We lost 11 that day. Everyone who was working," said Ladder 35 Capt. John Miles, who was with other firefighters early Tuesday morning setting up their annual breakfast for active and retired members as well as family members of those who died.
He said that even as the years pass, most of the families still come.
"It's good for us that they (the family members) are here and it's good for them that we are here," he said.
The official commemoration began at 8:39 a.m. ET at the National September 11 Memorial plaza, an area that once held the twin towers but now hosts two memorial pools dedicated to attack victims.
There was a moment of silence at 8:46 a.m. ET to mark when the first commercial plane struck the north tower. Houses of worship across New York City rang their bells in remembrance.
Throughout the morning family members of those who perished recited the victims' names. The names of all 2,983 victims from the twin towers and Pentagon attacks, and those on Flight 93, as well as those who died in the 1993 World Trade Center bombing, will be read aloud.
There was a moment of silence for each time a hijacked plane hit its target and one for when Flight 93 crashed in Shanksville, Pa. There also were moments of silence at the times that each of the twin towers fell.
President Obama was among the speakers Tuesday at an invitation-only remembrance for victims and family members of those killed at the Pentagon.
"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. Your loved ones will never be forgotten," Obama said. "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."
Afterward he shook hands with many of the family members attending the event.
"Eleven years ago, on a morning very much like this, terrorists attacked our symbols of our strength ... and took the lives of people from more than 90 countries," said Secretary of Defense Leon Panetta at the Pentagon. "Every year on 9/11, all of us take a moment to remember again where we were at that faithful moment," and the people who died at the World Trade Center, at the Pentagon and in a Pennsylvania field.
To the families, he said, "we are with you today and honored to be here today to remember your loved ones."
Before Obama headed for the Pentagon he and the first lady joined hundreds of White House staff members gathered on the South Lawn in the shade of the portico and in patches of sun. Most stood with their hands crossed in front of them.
A full Marine color guard emerged taking up a place on each side of a wide aisle for the president and Michelle Obama, who walked down the grassy strip as three bells tolled. They stood with heads bowed, facing the National Mall in the distance.
Obama and Mitt Romney will temporarily pull their largely negative campaign commercials off TV on Tuesday.
The Pentagon ceremony was held across from the National 9/11 Pentagon Memorial, where 184 metal benches memorialize the 59 crew and passengers and 125 people who were killed.
The ceremony will be similar to ceremonies in past years, though now there's a new project to look forward to, said Jerry Mullins, spokesman for the Pentagon Memorial Fund, which raised money and developed the memorial.
The fund is preparing for a visitor education center that organizers hope to complete by early 2015, Mullins said. Fundraising has already begun.
"The families are very proud of the memorial," Mullins said. "The recognition, and the pledge that was made 11 years ago -- "Never forget" -- is a great comfort to the families."
In Shanksville, Pa., people slowly started filing into the Memorial Plaza, including servicemen in uniform and others wearing red, white and blue.
Vice President Joe Biden spoke during the remembrance ceremony at the Flight 93 Memorial, where a hijacked plane crashed during the 2001 attacks.
Biden told families that he hopes with every passing year "you are able to sing more than you weep."
"What they did for the country is still etched in the minds of not only you, but millions of Americans," he said.
Of the four planes that terrorists hijacked, Flight 93 was the only one that did not hit its target. The plane was traveling from Newark to San Francisco when it was hijacked.
The 9/11 Commission said the terrorists likely wanted to crash the plane in Washington, D.C., targeting the White House or the U.S. Capitol, but passengers and crew fought back.
Instead of hitting a national landmark, the plane went down in a western Pennsylvania field killing everyone on board.
The service includes a reading of the 40 names of the Flight 93 passengers and crew, a ringing of the Bells of Remembrance and a wreathing laying.
Musical tributes, wreath laying, and additional activities continued through the afternoon.
New York's ceremony followed a last-minute breakthrough on a financial dispute that had halted progress on the Sept. 11 museum, and the commemoration itself was to be different: For the first time, elected officials did not speak at an occasion that has allowed them a solemn turn in the spotlight, but also has been lined with questions about separating the Sept. 11 that is about personal loss from the 9/11 that reverberates through public life.
For former New York governor George Pataki, this year's change ends a 10-year experience that was deeply personal, even as it reflected his political role. He was governor at the time of the attacks.
"As the names are read out, I just listen and have great memories of people who I knew very well who were on that list of names. It was very emotional," Pataki reflected by phone last week. Among his friends who were killed was Neil Levin, the executive director of the Port Authority of New York and New Jersey.
But Pataki supports the decision not to have government figures speak.
"It's time to take the next step, which is simply to continue to pay tribute," Pataki said.
The National Sept. 11 Memorial and Museum -- led by Mayor Michael Bloomberg as its board chairman -- announced in July that this year's ceremony would include only relatives reading victims' names.
The point, memorial President Joe Daniels said, was "honoring the victims and their families in a way free of politics" in an election year.
Some victims' relatives and commentators praised the decision. "It is time" to extricate Sept. 11 from politics, the Boston Globe wrote in an editorial.
But others said keeping politicians off the rostrum smacked of ... politics.
The move came amid friction between the memorial foundation and the governors of New York and New Jersey over financing for the museum. That friction abruptly subsided Monday, when Bloomberg and New York Gov. Andrew Cuomo announced an agreement that paves the way for finishing the $700 million project "as soon as practicable."
On Tuesday Obama's motorcade made an unscheduled stop at National Cemetery after the Pentagon ceremony. The president and Michelle Obama walked among the chalk-white markers in the bright sun and shade, pausing to place a "challenge coin" on one of the first graves.
The medallion is given by commanders as motivation to soldiers or to honor achievement. The Obamas talked for several minutes over the grave, which marked a collective memorial for victims of an Oct. 26, 2009, helicopter crash in Afghanistan.
After Obama placed several more coins on graves along the walk the motorcade rolled back to the White House.
By Laura Petrecca, Natalie DiBlasio and Oren Dorell, USA TODAY
Contributing: Carolyn Pesce in McLean, Va.; Associated Press.