In New York, the crowd gathered near the impressive Manhattan memorial museum fell silent several times for minutes of silence, marking the exact moments when the four planes hijacked by Islamist commandos had crashed, and when the two towers of the World Trade Center had collapsed in a deluge of steel and dust.
In the audience in New York, Vice President Kamala Harris listened to the very long list of victims' names. Joe Biden participated in another ceremony, at the Pentagon.
"The grief fades a little over time, but the permanent absence of my father remains just as present," said the son of Jon Leslie Albert, one of the victims of the attacks, after reading his father's name.
Another relative of the victim, calling on the political figures present to heal America's deep divisions, said that "we should not need another tragedy to unite our nation".
On September 11, 2001, 2,977 people died in the deadliest attacks in history, committed by the jihadist organization Al-Qaeda.
Two planes had struck the two towers of the World Trade Center in New York, a third had ripped open the Pentagon and a fourth, which appeared to be aimed at the Capitol or the White House, had crashed in a wooded area in Shanksville, Pennsylvania, after a passenger counterattack.
No one on board the four planes survived.
- Elizabeth II -
In Washington, President Joe Biden gathered at the Pentagon.
Looking solemn, one hand on his heart, he participated in a wreath laying ceremony near the building where one of the hijacked planes had crashed, killing 184 people.
"I know that for those who have lost someone, 21 years is both an eternity and such a short time," said the Democrat from the podium, in a fine rain.
Joe Biden has shared a message sent on September 11, 2001 by Queen Elizabeth II, who died on Thursday, to the American people.
"Sorrow is the price to pay for love," wrote the sovereign.
"The course of American history changed that day," continued the president.
But what did not change was "the character of this nation", "the sacrifices, the love, the generosity" of which the United States is capable, he hammered.
"Today is not about the past, but about the future," continued Joe Biden, calling on Americans to defend democracy, the guarantor of freedom that terrorists had wanted to "bury under fire, smoke and the ashes".
Jill Biden, First Lady of the United States, participated in a ceremony in Shanksville, Pennsylvania.
International leaders have also paid tribute to the victims of this attack which had marked the whole world.
"As we remember 9/11 and the innocent lives lost, we also remember the solidarity that held us together during those dark hours," Ukrainian President Volodymyr Zelensky tweeted.
Beyond the terrible toll of thousands of deaths and injuries, thousands more died in the following years from illnesses caused by toxic fumes from the collapse of the Twin Towers.