US President Barack Obama has paid tribute to the victims of the 11 September 2001 attacks.
Nearly 3,000 people died when the four planes crashed in New York, at the Pentagon and in a Pennsylvania field.
Speaking at the Pentagon on the eighth anniversary, Mr Obama vowed to "never falter" in pursuit of al-Qaeda.
As the ceremonies were held, it was reported that the US Coast Guard was conducting a training exercise on Washington DC's Potomac River.
Earlier reports suggested that shots were fired at a suspicious vessel on the river, but officials from the Department of Homeland Security denied this, telling the BBC the Coast Guard exercises were a daily occurrence.
'Sacrifices of thousands'
Thousands gathered for ceremonies in a square near New York's Ground Zero.
Remembrance services are being held around the country, with moments of silence being observed at the three sites.
Americans have been encouraged to contribute to a national day of service and in Afghanistan, US soldiers completed a 9.11km (5.5 mile) run to mark the day.
President Obama joined Defence Secretary Robert Gates at the Pentagon, where 184 people died, to meet members of victims' families and lay a wreath.
I greatly fear at some point we'll transition to turning it into Earth Day... and the remembrance part will become smaller
The names of the 40 passengers and crew will be read to mark the time of the crash - 1003.
In a break with tradition, the anniversary has also been designated a national day of service.
On Thursday, Mr Obama issued a statement urging Americans to take part in community work.
Conservation projects, aid packages for serving soldiers and the simple offering of work for free are among the undertakings made by members of the public.
However, some commentators and members of victims' families have expressed concern that the remembrance may lose its primary focus.
Debra Burlingame, whose brother died at the Pentagon, told Associated Press news agency: "I greatly fear at some point we'll transition to turning it into Earth Day where we go and plant trees and the remembrance part will become smaller."
About 1,000 US troops in Afghanistan marked the anniversary with a 9.11km run at the main Bagram base in Kabul. Two other bases also took part.
The 9/11 attacks sparked the US-led invasion in October 2001 to oust the Taliban and tackle al-Qaeda leaders living in Afghanistan.
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