America will fall silent at 8.46am local time (1.46pm BST) as the nation unites to mark the 10th anniversary of the September 11 attacks.
President Barack Obama will lead the country in remembrance, visiting all three sites where terrorist hijackers carried out their murderous acts, before delivering remarks at a commemorative event in Washington.
Nearly 3,000 people died in the 2001 atrocity, after al Qaida terrorists slammed passenger jets into the twin towers of the World Trade Centre in New York and the Pentagon in the nation's capital. A fourth plane crashed into a field in Pennsylvania after passengers stormed the cockpit in a desperate effort to prevent hijackers hitting a fourth target, Washington's US Capitol.
In New York, a commemoration ceremony will be held at the National September 11 Memorial located in the World Trade Centre site.
Moments of silence will be observed to coincide with the exact times that the hijacked planes hit in 2001, starting at 8.46am local time (1.46pm BST). Additional periods of reflection will take place at 9.59am (2.59pm BST) and 10.28am (3.28pm BST) to mark the moments that the two towers at the World Trade Centre fell.
During the ceremony, the names of those who died in the attack will be read, while bells are tolled across the city.
Among those present during the solemn ceremony are expected to be relatives of some of the 67 British victims of the attack. A separate event will take place later at the British Memorial Garden in New York's Hanover Square.
Some families of British 9/11 victims attended an early morning service of "remembrance and reconciliation" in the Grosvenor Chapel in central London on Sunday morning.
Survivor Courtney Cowart, who was nearly buried alive when the north tower of the World Trade Centre collapsed on top of her, described her fears when she returned to the site for a church service five days later. She told the congregation: "Entering the heart of darkness, I was terrified. We were dwarfed by immense wreckage looming around us. It was a landscape drained of all colour."
Deputy US ambassador to Britain Barbara Stephenson lit a candle during the service to symbolise the banishment of darkness and everlasting life. The Grosvenor Chapel is known as the "American Church" because of its strong ties to the nearby US embassy and history as a place of worship for American soldiers during the Second World War.
