Those across the United States on Sunday paused — some at formal commemorations, some in quiet remembrance — to honor their countrymen who died in the Sept. 11, 2001, terrorist attacks and to remember the day that changed the nation, and the world forever.

Thousands gathered at New York’s ground zero, site of the World Trade Center, and stood still in silence, some crying as they listened to the names of victims attacks read aloud.

“They were our neighbors, our friends, our husbands, wives, brothers, sisters, children and parents,” said New York Mayor Michael Bloomberg, who, along with President Barack Obama, helped lead the commemoration on the 10th anniversary of the attacks.

“We have asked their families to speak their names out loud, to remind each of us about the person we lost in New York, in Washington and in Pennsylvania. They each had a face, a story, a life cut short from under them.”

A moment of silence spread across New York City at 8:46 a.m. — the time when American Airlines flight 11 crashed into the North Tower of the World Trade Center. Houses of worship tolled their bells.

After Obama read a psalm, 167 pairs of family members began reading the names of those who perished. The reading was interrupted by another moment of silence at 9:03 a.m. — the time when United Airlines Flight 175 struck the South Tower.

The solemn ceremony at the site of the World Trade Center came amid a heavy security presence permeating the area in lower Manhattan, as authorities continued their search for possible plotters of another terrorist strike.

Information considered credible but unconfirmed indicated up to three attackers could be trying to use an explosives-laden vehicle for an attack in New York or Washington, according to various sources. Authorities stressed that while they were taking information of a possible attack plot seriously, there was no evidence so far that an actual terrorist operation was underway.

Roads near ground zero were blocked and police checkpoints for both vehicles and pedestrians surrounded the memorial, with backpacks checked by bomb-sniffing dogs and put through x-ray machines.

At the ceremony, Obama and first lady Michelle Obama stood behind bullet-proof protection. Former President George W. Bush, who was president at the time of the attacks, read a letter sent by Abraham Lincoln to a woman who lost five sons in the Civil War.

“I feel how weak and fruitless must be any word of mine which should attempt to beguile you from the grief of a loss so overwhelming,” Bush read, quoting Lincoln. “But I cannot refrain from tendering you the consolation that may be found in the thanks of the republic they died to save.”

Those who lost loved ones in the attacks stepped forward to read names. In all, 2,753 people died on two airplanes and on the ground when the planes slammed into the World Trade Center towers, causing their collapse. That total does not include the 10 al Qaeda hijackers on board the planes.

Parallel commemoration ceremonies also took place in Washington — where mourners observed a moment of silence at 9:37 a.m., the moment American Airlines Flight 77 struck the Pentagon, killing 184 people — and in Shanksville, Pennsylvania, where passengers aboard United Flight 93 are believed to have thwarted a plot to drive the plane through the U.S. Capitol dome and eventually caused the plane to crash in a field.

The total number of dead in New York, Washington and Pennsylvania was 2,977 people, not including 19 hijackers.

Moments of silence were observed at 9:59 a.m., the time the South Tower collapsed; at 10:03 a.m., the time of the Flight 93 crash in Shanksville; and at 10:28 a.m., the time the North Tower fell.

Those in the crowd included Joseph Conzo, who has been an EMT for 18 years. He recalled the moment the second tower collapsed, leaving him trapped beneath the smoldering rubble of a neighboring Marriott hotel.

“I still can’t believe it’s been 10 years,” he said. “I’ve got a range of mixed emotions.”

Conzo said he was in therapy for two years after the attacks, grappling with “survivors’ guilt.”

“It was the kind of thing that if you went right, you died, and if you went left, you lived,” he said. “I was left with the question, ‘Why me?'”

Sunday’s ceremony, to him, is about closure, he said, “Let’s get this finished and move forward.”

In Washington, the moment of silence was followed by remarks from Joint Chiefs of Staff Chairman Adm. Mike Mullen, Vice President Joe Biden and Secretary of Defense Leon Panetta.

Biden spoke on the nation’s efforts to combat al-Qaida in the wake of the attacks, referencing the May assassination of its leader Osama bin Laden.

“Those in this building that day knew what they were witnessing,” Biden said. “It was a declaration of war by stateless actors bent on changing our way of life, who believed these horrible acts, these horrible acts of terror directed against innocents could buckle our knees, could bend our will, could begin to break us and break our resolve. But they did not know us. Instead, that same American instinct that send all of you into the breach between the fourth and fifth corridors galvanized a new generation of patriots, the 9/11 generation.”

“No memorial, no ceremony, no words will ever fill the void left in your heart by [the victims] loss,” he told family members. “My prayer for you is that 10 years later, when you think of them … that it brings a smile to your lips before it brings a tear to your eye.”

At the ceremony, troops placed wreaths at the Pentagon memorial. En route to the Pentagon ceremony, Biden stopped by a Washington fire station.

Obama attended all three ceremonies Sunday. At Shanksville, he and the first lady laid a wreath at the Flight 93 memorial, prompting cheers of “USA,” from the crowd. The president also laid a wreath at the Pentagon memorial, and he and the first lady met with victims and victims’ relatives at all three sites. Obama will also speak Sunday night during the Concert for Hope at the John F. Kennedy Center

En route to Shanksville from New York, White House spokesman Josh Earnest told reporters the president was impressed by the 9/11 memorial.

“He also said that he was touched by the — sort of the serene setting of the reading of the names this morning,” Earnest said. “And he and the first lady were particularly moved by the families of those who were lost who participated in the ceremony, in particular the children.”

The World Trade Center Memorial will open to the public on Monday, but on Sunday was open only to victims’ family members and those attending the commemoration.

“We’re so proud of this memorial,” said Monica Iken, who lost her husband Michael in the attacks, founded the group September’s Mission and played an instrumental role in the construction of the site. “I can go see Michael. He’s home.”

She said she comes to the site to connect with her husband. “Every time I come here, I feel the energy. It’s powerful,” she said.

The name of Mark Zangrilli, 36, was one of the last to be read.

“It always seems like people leave,” said his sister, Linda Spinella, referring to the depleting crowd at the site. “I wish they would stay.”

She recalled her brother as a warm person with “a really contagious laugh. He loved his kids and he was good to his parents.”

“It’s a terrible loss,” she said. “Too many people lost too many people.”

Commemoration ceremonies — some of them impromptu — were also taking place elsewhere.

At Fort Drum, NY — home to the 10th Mountain Division, which has led the U.S. war effort in Afghanistan — silence fell as bells tolled and a bagpiper played “Amazing Grace.”

Soldiers and their families gathered in front of a memorial that says, “Lest we forget,” etched with images of the three sites and the division’s insignia.

In Washington, hundreds gathered for an interfaith prayer vigil.

Commemorations were held at sites nationwide, but others chose to mark the anniversary in their own way.

“Seeing the horror of that day changed me,” said CNN iReporter Jason Asselin of Iron Mountain, Michigan. “I always thought of our flag as just a flag … I know this sounds strange but I never gave the flag the respect it deserved. It took a tragedy for me to feel this way. Many people died that day because of our flag … When I look at our flag today, I see policemen, firemen and military personnel. These are the people who will protect us from the evils in the world.”

Another iReporter, Bryan Bonner of Alexandria, Va., said the attacks prompted him to leave his position at the Federal Reserve Board and join the Transportation Security Administration, where he works on national security issues. Bonner said his mother was in New York that day and watched the towers fall and people falling from the building. “Every anniversary day she relives those moments, but at least now she knows that her son is doing something about it,” he said.

Kathi Cordsen sent in an iReport of steel beams from ground zero on display Sunday at the Richard Nixon Presidential Library in Yorba Linda, Calif. “I wanted to see this,” she said. “I wanted to touch the beams.”

“This anniversary just brings back all the memories of that day,” she said. “The horror of an accident, then the terror I felt when the second plane hit, that’s when everyone knew it was no accident.”

After the program at the World Trade Center, the Port Authority of New York and New Jersey held an interfaith remembrance service in lower Manhattan to honor 84 of its employees who died in the New York attacks, as well as the six victims of the 1993 World Trade Center bombing.

At New York’s John F. Kennedy International airport, there was little passenger traffic on Sunday, but people gathered around televisions at the airport to watch the ceremonies. At 8:46 a.m., during the first moment of silence, the airport grew quiet as passengers stopped and a police officer removed his hat.

“Today the buses are running, the birds are chirping,” a pilot told passengers before takeoff, according to CNN’s Laurie Segall, who was aboard. “It’s a normal day. We must all be proud to be Americans today.”

“It’s extremely emotional,” said Felicia Cappo of Jackson, NJ, who was among those gathered at the World Trade Center site. She was holding a laminated photo of her brother, Gary Frank, who died in the attacks. “It’s hard to believe 10 years have gone by … it will always feel like yesterday.”

She said she makes an annual visit to Ground Zero. “No remains were ever found, so this is where we come to pay tribute to my brother.”

Patric Tengelin, 37, came from Mamlo, Sweden, to New York with his mother and sister to honor his brother David Tengelin, 25, who died in the World Trade Center.

“I can mourn him anywhere,” Tengelin said, “but it’s important to come here too.”

“I remember his kindness,” he said of his brother.”He treated everyone with respect and he had a heart of gold … I loved how to came here to pursue the American dream. He showed us you have to go after your dreams no matter what. He’s a big inspiration for all of us.”

Tengelin was carrying a placard with his brother’s name and picture on it.

Rosaria Reneo of Staten Island remembered she had just dropped her children off at school that day when she got a phone call that a plane had hit the World Trade Center, where her sister, Daniela Notaro, was working on the 92nd floor of the North Tower. She never heard from Notaro again, despite making frantic calls to her sister’s cell phone.

“We come here every year,” Reneo said, clutching a picture of her sister. She said in spite of the increased security, she felt compelled to come Sunday. “My kids didn’t want me to come,” she said. “And I did worry a bit.”

Firefighter Richard Batista, 38, was in the North Tower when the South Tower collapsed. His unit worked its way down a smoke-filled staircase and outside the building before the second collapse.

“My captain, in his infinite wisdom, decided that there were too many crews up there and told us to meet outside and wait for further instructions,” he said.

Moments later, the North Tower fell in a dark plume of smoke, ash, dust and debris. “We took shelter in a parking garage, and that’s what saved us,” Batista said. “With a rescue rope, we were able to slowly pull and guide ourselves out of the rubble.”

A decade later, he said his wish is to move on.

“I’ve gotten to the point where I’m numb to it,” he said. “And while I have all the respect for the families who lost loved ones, these types of things today make it more difficult.”

Louis Jerez of Pasadena, Md., was carrying a poster with several photos of his cousin, Angel Louis Juarbe Jr., a firefighter who died that day.

“This is hard. It’s not easy,” Jerez said. “Every year, my chest feels tight … you don’t heal from this kind of loss.”

Bronx resident Miguel Flores, 37, was among dozens of paramedic units who charged toward the burning towers. He arrived just as the South Tower crumbled and helped set up a triage unit to help separate the dead from the dying.

“The corner Burger King was converted into a triage center,” Flores said. “The Brooks Brothers was turned into a morgue.”

As rescue workers dug through the rubble, paramedics labored over those injured.

He said he lost 13 percent of his lung capacity when a dust cloud enveloped the site. Ten years later, he has physically recovered, he said.

“It’s a numbing feeling,” he said. “Being back here, you start to relive that a little. We all have dealt with post-traumatic stress.”

Eight miles north of ground zero, New York firefighters held their own memorial at Riverside Park. While the event has been held annually since 9/11, this event was by far the largest, with thousands of firefighters from around the nation in attendance as well as some from as far away as Germany and Australia. The firefighters stood in a formation line that stretched for blocks.

Firefighter Peter Regan was at Riverside Park although he could have gone to the World Trade Center ceremony, as he lost his father, Donald Regan, in the attacks. He said he’d rather be with the rest of the firefighters.

“I absolutely loved today’s service,” said Lt. George Ricco Diaz of the Bronx. “What’s outstanding is the turnout of brothers, brother firefighters from around the world. We got all kinds of firefighters here from all around the world, and that’s an impressive sight to see.”

In addition to the commemoration at Riverside Park, a solemn memorial service at St. Patrick’s Cathedral in New York included the recitation of the names of 343 firefighters who died at ground zero.

Patrick Mate Lyons, who was born Oct. 7, 2001, read an open letter to his father, Patrick, one of those killed.

“I want you to know that Mommy is doing a great job of loving me and raising me in a happy home,” Patrick said. “I play flag football in the same league as you, and in the same position as you, as quarterback. In baseball, I pitch, just like you did. I really like it when people compare me to you.”

Mark Tillman, a 30-year Air Force veteran who piloted Air Force One, was among those at the New York ceremony. The plane, which carried the classified code name “Angel,” took off carrying then-President Bush from the tarmac in Sarasota, Fla., that day, traveling to Barksdale Air Force Base in Louisiana, where Bush taped an address to the nation.

“The challenge for us was that we didn’t know what the threat was,” Tillman recalled. He said he called in a fighter jet escort to accompany the plane, and later received a call from then-Vice President Dick Cheney.

“Angel was next,” Cheney told Tillman. “To me,” Tillman said, “it was an act of war and the vice president was telling me that the president could be the next target.”

Air Force One returned to Andrews Air Force Base in Maryland about 6 p.m. that night, after Washington had been secured.

Tillman said that Sunday’s ceremony represents solemn remembrance to him, but also signals what will continue.

“September 11 is a day in history, but it’s not World War I or World War II,” he said. “This battle hasn’t ended.”

“It’s the first time it feels right,” said actor Denis Leary, who has helped champion health coverage for first responders affected by the toxic plume which enveloped much of lower Manhattan following the attacks. “What they did on 9/11 is an extreme version of what they do and the risks they take every day.”

“I have no patience for those who have prevented first-responder care,” he said, referencing controversy that swirled around whether the Zadroga bill, a recently piece of legislation that provides health care to those affected by 9/11, will also include cancer coverage.

“They should be the first guys we check off the list,” he said.

Celeste Pollicino, 22, of Plainview, NY, wore an “I love New York” T-shirt and held a framed photo of her father, Steve Pollicino, who worked as a bond trader in the World Trade Center.

“I remember his smile,” she said. “He was just fun… Now I feel like I’ve lived two different lives, before and after 9/11.”

She said her father’s death “makes me appreciate everything, every day. I’m more easygoing now. I don’t let little things bother me.”

Her cousin, Michelle Angeline, said Celeste Pollicino is “so much like her dad in the best way possible.”

CNN’s Moni Basu and Dana Garrett contributed to this report.