US President Joe Biden has urged 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 on Saturday.
Joe Biden will begin with a ceremony at 08:30 (12:30 GMT) in New York at the site where the World Trade Center’s Twin Towers once stood.
He is not expected to deliver any speeches, but in his address on Friday, he said: “No matter how much time has passed, these commemorations bring everything painfully back as if you just got the news a few seconds ago.”
“We learned that unity is the one thing that must never break,” he added.
Queen Elizabeth has sent a message to Mr Biden saying her visit to the World Trade Center in 2010 was “held fast in my memory”.
“My thoughts and prayers, and those of my family and the entire nation, remain with the victims, survivors and families affected, as well as the first responders and rescue workers called to duty,” the Queen said.
Among the dead in the 11 September attacks were 67 Britons.
PM Boris Johnson said the attacks had failed “to shake our belief in freedom and democracy”.
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.
Another plane struck the Pentagon, just outside the US capital, Washington DC, and a fourth crashed in a field in Pennsylvania after passengers fought back.
President Biden will visit all three attack sites, along with the first lady, Jill Biden.
There will 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.
The commemorations come at a difficult time for the president, who has faced strong criticism in recent weeks over the US withdrawal from Afghanistan, which ended a US presence that began less than a month after the 11 September attacks.