The loved ones gathered again in Lower Manhattan for moments of silence and the peals of the bells. They paid tribute to those they lost 20 years earlier and listened as Bruce Springsteen sang about memory and loss.

In Pennsylvania, hundreds gathered in a clearing where a hijacked plane had crashed to hear a former president honor their relatives’ sacrifice. In Chicago, firefighters too young to remember Sept. 11 climbed thousands of steps to honor emergency responders who had not lived past it. In Nebraska, children sat on the shoulders of their parents and waved American flags to honor the victims of a tragedy that, to them, has only been history.

For two decades, Americans have mourned the attacks of Sept. 11 that killed nearly 3,000 people, a loss so deep it shook the United States to its core. But even as time has passed, and the horrific day has moved from fresh memory into the chronicles of history, the people who gathered across the country and the globe said the wounds from 9/11 have remained fresh.

“Twenty years feels like an eternity,” said Lisa Reina, her voice quavering as she held up a photo of her husband, Joseph Reina Jr., who was at work in the north tower of the World Trade Center when the first plane struck. “But yet it still feels like yesterday.”

The ceremonies were part of a full day of tearful commemoration across the United States and the world, which watched in shock 20 years ago as passenger jets hijacked by Qaeda terrorists slammed into the towers of the World Trade Center, the Pentagon and a field near Shanksville, Pa., the deadliest attack ever on American soil.

The anniversary arrives as the United States is in the throes of another grim historic event: the coronavirus pandemic, which has claimed more than 656,000 lives, upended the economy and exposed gaping fault lines in the fabric of American life. In the last week, as many Americans have died of complications from the virus every two days as perished in one fell swoop on Sept. 11.

And the country has only just closed the chapter on a costly and devastating war that sprang from 9/11’s wake: a 20-year occupation in Afghanistan that began as a hunt for the terrorists who oversaw the attacks and ultimately ended with 170,000 lives lost — more than 2,400 of them Americans — and the same Taliban militants in power there. More than 100,000 Iraqis and 4,400 Americans were killed in the war in Iraq, also waged in the aftermath of Sept. 11.

To honor the milestone, President Biden, who as a senator 20 years ago sought desperately to soothe a panicking country and who later voted to authorize the war in Iraq, visited the site of each attack for ceremonies honoring the lives lost there.

In a video message, Mr. Biden, who now leads the country in one of its most divided moments, celebrated the unity that the United States experienced in the wake of the attacks.

“We saw national unity bend,” he said. “We learned that the unity is the one thing that must never break.”

Former President George W. Bush, who attended a memorial ceremony near Shanksville, also praised the spirit of unity after the attacks. But he also pointedly drew a contrast with the country’s current politics, connecting the Qaeda terrorists who hijacked planes to domestic extremism.

Speaking eight months after the U.S. Capitol was stormed by supporters of former President Donald J. Trump, Mr. Bush said that while there was “little cultural overlap between violent extremists abroad and violent extremists at home,” both groups showed a “disregard for human life” and “determination to defile national symbols.”

Mr. Trump on Saturday did not attend the memorial services in New York or Pennsylvania. He instead spoke to police officers at the 17th Precinct station house in Midtown Manhattan, where he spent time attacking Mr. Biden and other Democrats for their handling of the American withdrawal from Afghanistan, but said comparatively little about Sept. 11.

At the 9/11 memorial plaza in Lower Manhattan during the morning, Mr. Biden and the first lady, Jill Biden, stood shoulder to shoulder with their Democratic predecessors, Barack and Michelle Obama and Bill and Hillary Clinton, who was a New York senator 20 years ago.

Nearby was Rudolph W. Giuliani, a Trump ally who was the mayor of New York during the attacks and has since become one of the most vociferous critics of Mr. Biden, Mrs. Clinton and Mr. Obama.

All of them watched as relatives of the victims read the names of the dead, an annual recitation that paused for moments of silence marking the times when the hijacked planes hit their targets and when the twin towers eventually fell. As the plaza fell silent, bells rang.

Many of those who read names at the memorial were children, either born after the attacks or too young to remember the friends and family members who died. Ms. Reina, whose husband worked at Cantor Fitzgerald, the bond trading firm that lost 658 employees in the attack, was pregnant on 9/11. Her son is part of an entire generation that has been born in the shadow of that day and has only received its legacy secondhand.

Ariana and Briana Mendoza, 13, came to Lower Manhattan from the Bronx with their sister, Dephaney, to pay their respects. “I was only 2 when it happened,” Dephaney, 22, said. “But I have learned a lot about it, and now I am teaching them.”

Nearby, Luis Gonzalez, 41, of Staten Island, stood staring up at One World Trade Center, the tower built upon the ruins of ground zero. He carried a poster of the old twin towers that now hangs in his bedroom. “I come out of respect,” he said.

Many others went to smaller but no less emotional events across the country for similar reasons. At the stair-climbing event in Chicago, Marisa Price, a firefighter from Westmont, Ill., said she felt a deep connection to the emergency workers who gave their lives during rescue efforts.

“A lot of them were my age at the time and made the ultimate sacrifice,” Ms. Price, 25, said. “It’s something we’re all willing to do but don’t want to do.”

At one of the day’s marquee college football games, the Ohio State University marching band performed a patriotic tribute during halftime, moving through formations that included an American flag, the Statue of Liberty and a bald eagle.

Memorials were also held across the globe. Outside Buckingham Palace in London, during the changing of the guard, the guardsmen’s band played “The Star-Spangled Banner,” as it had after Sept. 11, 2001. At the NATO headquarters in Brussels, the secretary-general stood in front of a piece of twisted metal from the World Trade Center for a moment of silence.

Mr. Bush and Vice President Kamala Harris both took part in a ceremony at the Flight 93 National Memorial in Pennsylvania, during which the names of the 40 passengers and crew members on the plane were read aloud.

Ms. Harris said that the Sept. 11 attacks had demonstrated how “fear can be used to sow division.” She stressed that America’s diversity was its greatest asset and encouraged the country to reflect on the sacrifice made by those who died.

“On this 20th anniversary, on this solemn day of remembrance, we must challenge ourselves to, yes, look back,” Ms. Harris said. “For the sake of our children. For the sake of their children. And for that reason, we must also look forward. We must also look toward the future. Because in the end, that is what the 40 were fighting for: their future, and ours.”

Hours later, Mr. Biden and Dr. Biden arrived to lay a wreath at the memorial. Then, holding hands, they strode quietly toward a boulder marking the area where the plane hit the ground and spent time with the families of victims.

The president and the first lady then traveled with Ms. Harris and her husband to another wreath ceremony at the Pentagon in Arlington, Va., where 184 people were killed after a plane hit the building’s west side.

Earlier on Saturday, the Department of Defense unfurled a large American flag on the side of the building and held a memorial ceremony there.

“The hallways that we tread were the ones where so many of them walked,” said the secretary of defense, Lloyd J. Austin III. “It will always be our duty to fulfill their missions and live up to their goodness and to stand guard over this democracy.”