Thousands were invited to attend ceremonies in New York, while strict restrictions limited the number of attendees in Washington, DC.
Americans have commemorated 9/11 amid a new national crisis – the coronavirus pandemic.
In New York, victims' relatives gathered for split-screen remembrances, one at the September 11 memorial plaza at the World Trade Center and another on a nearby corner, set up by a separate organisation.
The Stephen Siller Tunnel to Towers Foundation objected to the memorial's decision to forgo a longstanding tradition of having relatives read the names of the dead, often adding poignant tributes. Memorial leaders said they made the change as a coronavirus-safety precaution on the 19th anniversary of the attacks.
Kathy Swift arrived early at the alternate ceremony a few blocks away, wearing a T-shirt honouring her slain brother, Thomas Swift, who worked in finance.
"We still have to remember," said Swift, 61. “The whole country’s going downhill. It’s one thing after another, and now with Covid. I’m glad they’re still having this, though.”
President Donald Trump addressed a ceremony at the Flight 93 National Memorial near Shanksville, Pennsylvania.
"The heroes of Flight 93 are an everlasting reminder that no matter the danger, no matter the threat, no matter the odds, America will always rise up, stand tall, and fight back,” the Republican president said, recalling how the plane's crew and passengers tried to storm the cockpit as the hijackers headed for Washington.
Biden pays respects
Biden planned to pay respects at the election battleground state's Flight 93 memorial in the afternoon. Earlier, he attended the observance at the 9/11 memorial in New York, exchanging an elbow bump with Vice President Mike Pence before the ceremony began with the usual tolling of a bell.
Biden offered condolences to a woman he spotted crying in the crowd of hundreds, Amanda Barreto, who lost her aunt and godmother in the attacks.
Barreto, 27, said Biden "wanted to let me know to keep the faith" and "wanted me to say strong," telling her he understood what it meant to lose a loved one. His first wife and their daughter died in a 1972 car crash, and his son Beau died of brain cancer in 2015.
Biden didn't speak at the ceremony, which has a longstanding custom of not allowing politicians to make remarks.
Pence reads scripture
Pence went on to the Tunnel to Towers Foundation ceremony, where he read the Bible's 23rd Psalm, and his wife Karen read a passage from the Book of Ecclesiastes.
“For the families of the lost and friends they left behind, I pray these ancient words will comfort your heart and others,” said the vice president, drawing applause from the crowd of hundreds.
READ MORE: Who is Khalid Sheikh Mohammad - the alleged 9/11 conspirator?
Ceremonies impacted by Covid-19
In short, the anniversary of 9/11 is a complicated occasion in a maelstrom of a year, as the US grapples with a health crisis, searches its soul over racial injustice and prepares to choose a leader to chart a path forward.
Still, families say it's important for the nation to pause and remember the hijacked-plane attacks that killed nearly 3,000 people at the World Trade Center, at the Pentagon in Washington and near Shanksville on September 11, 2001 – shaping American policy, perceptions of safety and daily life in places from airports to office buildings.
Around the country, some communities cancelled 9/11 commemorations because of the pandemic, while others went ahead, sometimes with modifications.
The Pentagon's observance was so restricted that not even victims’ families could attend, though small groups can visit the memorial there later in the day.
At the New York memorial, thousands were still invited. But they heard a recording of the names issued from speakers spread around the vast plaza, a plan that memorial leaders felt would avoid close contact at a stage but still allow families to remember their loved ones at the place where they died.
But some felt the change robbed the observance of its emotional impact. The Tunnel to Towers Foundation arranged its own, simultaneous ceremony a few blocks away, saying there was no reason that people couldn't recite names while keeping a safe distance.
Reverence for the dead “requires that we read these names out loud, in person, every year,” said foundation chair Frank Siller, whose brother Stephen was a firefighter.
The readers stood at podiums that were wiped down between each person.
The two organisations also tussled over the Tribute in Light, a pair of powerful beams that shine into the night sky near the trade center, evoking its fallen twin towers.
The 9/11 memorial initially cancelled the display, citing virus-safety concerns for the installation crew. After the Tunnel to Towers Foundation vowed to put up the lights instead, the memorial changed course with help from its chair, former mayor Mike Bloomberg, and Governor Andrew Cuomo.
Tunnel to Towers, meanwhile, arranged to display single beams for the first time at the Shanksville memorial and the Pentagon.
Over the years, the anniversary also has become a day for volunteering. Because of the pandemic, the 9/11 National Day of Service and Remembrance organisation is encouraging people this year to make donations or take other actions from home.