Americans have held a series of solemn ceremonies to mark six years since the 11 September 2001 attacks on the US.
The families of those killed in New York laid flowers at Ground Zero
Nearly 3,000 people were killed when four planes were hijacked and flown into New York's World Trade Center, the Pentagon and a field in Pennsylvania.
Church bells tolled at 0846 local time to mark the moment when the first Trade Center tower was hit.
A message purportedly from al-Qaeda head Osama Bin Laden has been released, praising one of the hijackers.
For the first time, much of the New York ceremony took place away from the World Trade Center site - known as Ground Zero - because of construction there.
Those taking part gathered instead at a nearby park, under overcast skies.
The names of the New York victims were read out as in previous years but for the first time by the surviving firefighters and other emergency workers on duty at the time.
Some of those reading out the names were overcome with emotion as they came to list fallen colleagues.
The ceremony included two pauses to mark the moments when the two planes crashed into the twin towers, and two more to mark the moments when the buildings collapsed.
Relatives of the 2,749 people killed in New York by 10 of the hijackers filed down a ramp into the World Trade Center site to lay flowers.
New York's Mayor Michael Bloomberg described 11 September 2001 as "the day that tore across our history and our hearts".
New York Mayor Michael Bloomberg paid tribute to the victims
He said: "We come together again as New Yorkers and as Americans to share a loss that can't be measured."
Rudy Giuliani, the city's mayor at the time, has attracted criticism for using his performance in the weeks after the 2001 attacks in his campaign to be the Republican Party's presidential nominee.
He said his appearance at the ceremony was not intended to be political.
"It was a day with no answers, but with an unending line of people who came forward to help one another," he said in brief remarks.
President George W Bush, who in past years has laid a wreath at the site and made a speech, observed a moment of silence at the White House.
President George W Bush marked the anniversary at the White House
Defence Secretary Robert Gates hosted a memorial service at the Pentagon for relatives of the 184 people killed by the five hijackers aboard American Airlines flight 77.
He paid tribute to those killed and vowed to defend the American people and their values from any other threat.
"The enemies of America, the enemies of our values and our liberty, will never again rest easy because we will hunt them down relentlessly and without reservation," he said.
A further ceremony was held in Shanksville, Pennsylvania, where United Airlines flight 93 crashed after some of the 40 passengers and crew rushed the four men who hijacked that plane.
A memorial event was also held at the main US airbase in Afghanistan.
A new message purported to be from Bin Laden has been released to coincide with the anniversary.
In it, a still photograph of the al-Qaeda leader is shown while his voice is heard praising the role of Waleed al-Shehri, one of the 19 hijackers.
Bin Laden's new message praises a 9/11 hijacker
It comes just a few days after the release of Bin Laden's first video for three years.
In the new message he wears the same beige cloak and dark, trimmed beard as in last week's video.
The new video also shows footage of al-Shehri, recorded shortly before his death, speaking about what motivated him to help hijack American Airlines flight 11, the first of the planes flown into the twin towers.
It is the sixth video released by al-Qaeda featuring the hijackers.
BBC security correspondent Gordon Corera says it is part of a regular cycle of al-Qaeda releasing videos to coincide with the 9/11 anniversary.