WASHINGTON - President Barack Obama set aside his Syrian agenda Wednesday to lead the nation in observing the 12th anniversary of 9/11, honoring those who perished in the worst terror attack in U.S. history.

“Together we pause and we pray,” Obama said at a ceremony at the Pentagon just outside Washington. “We pray for the memory of all those taken from us. Our hearts still ache for the futures snatched away.”

Almost 3,000 people were killed Sept. 11, 2001, when al-Qaida terrorists hijacked four commercial airliners and crashed them into the World Trade Center towers in New York City, the Pentagon and a field in rural Pennsylvania.

Obama, joined by Defense Secretary Chuck Hagel and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, laid a wreath to honor Pentagon victims, their families and first responders. At the Pentagon, 59 passengers and crew members and 125 people on the ground died.

Obama said the nation reaffirms the “values and virtues” of those who perished and that the nation will remain resilient in their honor.

“There’s no trouble we can’t endure and no calamity that we can’t overcome,” he said. Americans will “carry on, no matter how dark the night nor how difficult the days.”

The president also paid tribute to the four Americans killed a year ago in an attack on a U.S. compound in Benghazi, Libya, asking the country to pray for those who “serve in dangerous posts” even after more than a decade of war.

Life in Lower Manhattan resembled any ordinary day Wednesday as workers rushed to their jobs in the muggy heat, but time stood still at the World Trade Center site while families wept for loved ones who perished 12 years ago.

“Has it really been 12 years? Or 12 days? Sometimes it feels the same,” said Michael Fox, speaking aloud to his brother, Jeffrey, who died in the South Tower. “Sometimes I reach for the phone so I can call you and we can talk about our kids like we used to do every day.”

On the memorial plaza overlooking two reflecting pools in the imprint of the twin towers, relatives recited the names of the nearly 3,000 people who died on 9/11. They also recognized the victims of the 1993 bombing at the trade center.

Bells tolled to mark the planes hitting the towers and the moments when the skyscrapers fell.

Former Gov. George Pataki, New Jersey Gov. Chris Christie and others were in attendance. As happened last year, no politicians spoke. Mayor Michael Bloomberg watched the ceremony for his final time in office.

At a remembrance service at St. Peter’s Roman Catholic Church, in the shadow of where the World Trade Center once stood, Bloomberg spoke of the 84 Port Authority employees killed on 9/11.

“On that terrible day, we were united in prayer and compassion for all of you who lost loved ones,” the mayor said. “As we woke up this morning, our first thoughts were with you as well.”

The anniversary arrived amid changes at the Flight 93 National Memorial in Shanksville, Pa., where construction started Tuesday on a new visitor center.

On Wednesday, the families of the passengers and the crew aboard United Flight 93 recalled their loved ones as heroes for their unselfish and quick actions. The plane was hijacked with the likely goal of crashing it into the White House or Capitol, but passengers tried to overwhelm the attackers and the plane crashed into the field. All aboard died.

“In a period of 22 minutes, our loved ones made history,” said Gordon Felt, president of the Families of Flight 93, whose brother, Edward, was a passenger.

In Washington, the president, joined by first lady Michelle Obama and Vice President Joe Biden and his wife, Jill, along with White House aides, gathered on the South Lawn of the White House, where flags were at half-staff.

They stood silent at 8:46 a.m., the time in New York 12 years earlier when American Airlines Flight 11 smashed into the North Tower of the World Trade Center. Taps were played at the end of the moment of silence.

Members of Congress also marked the day by gathering outdoors in a solemn ceremony on Capitol Hill.

Outside Washington, hundreds of people gathered for a short, simple ceremony at an Arlington County plaza. First responders from the county were among the first on the scene at the Pentagon on 9/11.

Around the world, thousands of volunteers pledged to do good deeds, honoring an anniversary that has been designated a National Day of Service and Remembrance.

“On September 11, 2001, no matter where we came from, what God we prayed to, or what race or ethnicity we were, we were united as one American family,” Obama said in a proclamation issued Tuesday, in which he encouraged Americans to get involved in their communities. “May the same be said of us today, and always.”

After attending remembrances, Obama took part in a public service event, helping pack fresh fruit, muffins and other breakfast items at a food distribution center in Washington for people with cancer, HIV/AIDS or other afflictions.

The group Food & Friends delivers about 1 million meals to 3,000 people in Maryland, Virginia and the District of Columbia, spokesman Christopher Copley said.

Meanwhile, Michelle Obama drew a pair of construction paper hands for a project with military children during a visit to a USO center at the Army’s Fort Belvoir in Virginia.

The first lady said she wanted to highlight the organization’s work on behalf of wounded service members and their families.

Afterward, she toured the Intrepid Spirit center, also at Fort Belvoir. The facility was dedicated Wednesday and provides outpatient care to service members and veterans with traumatic brain injuries, post-traumatic stress disorder and related conditions. She met privately with wounded service members and their families and participated in a round-table discussion with caregivers.

The Obama administration said in a statement Wednesday that it’s taken several steps to prevent another attack like 9/11 in the U.S., citing a review of security procedures over the past several months led by Lisa Monaco, the president’s assistant for homeland security.

The steps weren’t identified, though the review covered global threats, including those from the Arabian Peninsula that led to the closing of 19 U.S. diplomatic posts last month.

With the nation once again debating America’s role in the world and the wisdom of launching military strikes, the memorial ceremonies Wednesday offered not just an occasion to pay tribute but also a moment to take stock.

Edwin Aviles, 41, who lives in Brooklyn and was working near ground zero, said time had done little to ease his sense that there are enemies looking to harm New York.

“I don’t think anything has changed,” he said. “Just like then, just like now we got to stay on point. Got to stay on top of everyone else. Anything can happen at any given moment.”

At the Pentagon, Obama noted the continuing threat.

“Let us have the strength to face the threats that endure, different though they may be from 12 years ago, so that as long as there are those who would strike our citizens, we will stand vigilant and defend our nation,” he said.

In Lower Manhattan, healing the physical scars of 9/11 has been slow, but progress is now evident. In the past year, the city celebrated the topping of 1 World Trade Center, its spire rising 1776 feet.

By next year’s anniversary, a 9/11 museum is expected to be open beneath the memorial plaza in New York City. While the memorial honors those killed, the museum is intended to present a broader picture, including the experiences of survivors and first responders.

“As things evolve in the future, the focus on the remembrance is going to stay sacrosanct,” memorial President Joe Daniels said.

Douglas Hamatie, whose 31-year-old cousin Robert Horohoe worked for Cantor Fitzgerald and died on 9/11, drew applause from the crowd at the memorial site Wednesday when he declared that the day should become a national holiday. “The kids today, they know when the next iPhone’s coming out, and they know when the next Justin Bieber concert is, but they don’t know enough about 9/11,” he said.“So let’s change that, please.”

The organizers have said they will always keep the focus on the families, and that was evident this year as a small crowd gathered on the tree-laden plaza - only friends and family of the victims were allowed.

Information for this article was contributed by Roger Runningen of Bloomberg News; by Jim Fitzgerald, Meghan Barr, Verena Dobnik, Jennifer Peltz, Colleen Long, Nedra Pickler, Matthew Barakat, Darlene Superville and Kevin Begos of The Associated Press; by Michael Muskal and Tina Susman of the Los Angeles Times; and by Marc Santora and Ariel Kaminer of The New York Times.

Front Section, Pages 1 on 09/12/2013