Official and individual events are under way across the US to remember the day when four hijacked airliners were flown into sites in New York, Virginia and Pennsylvania.
US President Barack Obama is leading tributes to the nearly 3,000 people killed in the attacks eight years ago.
He met relatives of those killed in the attacks after paying tribute to the victims in a speech at the Pentagon.
President Obama said: 'No turning of the seasons can diminish the pain and the loss of that day.
'No passage of time, no dark skies can ever dull the meaning of this moment.
'In the defence of our nation we will never waver, in pursuit of Al-Qaeda and its extremist allies we will never falter.'
Earlier, he led a moment's silence at 8.46am (1.46pm Irish time), the exact time that the first hijacked plane hit the north tower at the World Trade Center.
US Vice President Joe Biden attended the commemorative events in New York.
At the site where the towers once stood, relatives of those killed joined volunteers from across New York City to read the names of the victims.
The public reading was paused four times to mark the moments when American Airlines Flight 11 and United Airlines Flight 175 hit the buildings, and the moments when the two towers collapsed.
At nightfall, two beams of light will shoot skyward from the site.
A ceremony was also held in a Pennsylvania field where a fourth plane (United Airlines 93) crashed short of its target.
A US Coast Guard training exercise set off a security scare in Washington's Potomac River this afternoon.
Police and federal law enforcement officials said the incident was actually a training exercise.
It prompted reports that shots had been fired at a boat in the river near the Pentagon.
President Obama and Defence Secretary Robert Gates had been attending a ceremony at the Pentagon, south of the city on the Virginia side of the river.
'They're in a training exercise and we didn't know it. Somebody reported that they had shot at somebody. That was part of the training exercise,' said Mike Libby of the US Park Police.
A FBI official later confirmed that no shots had been fired.