US President Joe Biden has called for unity as his country remembers the victims of the 2001 11 September attacks.
In a video released on the eve of the 20th anniversary, he paid tribute to the 2,977 people who lost their lives.
“We honour all those who risked and gave their lives in the minutes, hours, months and years afterwards,” Mr Biden added, speaking of the emergency workers who responded to the attacks.
Commemoration events are due to take place today.
There will also be six moments of silence to correspond with the times the two World Trade Center towers were struck and fell, and the moments the Pentagon was attacked and Flight 93 crashed.
“No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few seconds ago,” the president said.
He acknowledged the “darker forces of human nature – fear and anger, resentment and violence against Muslim Americans” which followed the attacks, but said that unity had remained the US’ “greatest strength”.
“We learned that unity is the one thing that must never break,” he added.
British Prime Minister Boris Johnson meanwhile said the attacks had failed “to shake our belief in freedom and democracy”. Among the dead in the 11 September attacks were 67 Britons.
The attacks, which were planned by al-Qaeda from Afghanistan, saw four US passenger jets seized by suicide attackers – two of which were flown into the Twin Towers of the World Trade Center in New York.
Another plane crashed into the Pentagon, just outside the US capital, Washington DC, and a fourth plane crashed in a field in Pennsylvania after passengers fought back.