Commemorations of the seventh anniversary of 9/11
US President George W Bush has led commemorations of the seventh anniversary of the 9/11 attacks, which left nearly 3,000 people dead.
Moments of silence were held at the times four hijacked passenger planes hit the Twin Towers in New York, the Pentagon and a Pennsylvania field.
In Washington, Mr Bush dedicated a new memorial at the Department of Defense to 184 people killed there.
"The worst day in America's history saw some of the bravest acts," he said.
"Since 9/11 our troops have taken the fight to the terrorists abroad so we do not have to face them here at home," he added.
"Thanks to the brave men and women and all those who work to keep us safe there has not been another attack on our soil in 2,557 days."
The attacks, which led President Bush to declare the US "war on terror", are regarded as the defining moment of the president's time in office.
In downtown Manhattan, thousands of people gathered as relatives of victims from more than 90 countries read out a roll call of the 2,751 people killed in New York.
City Mayor Michael Bloomberg opened the memorial event, describing 9/11 as a day that "lives forever in our hearts and our history".
Silences were observed at the moments each of the Twin Towers was struck and fell.
Barack Obama and John McCain, the Democratic and Republican nominees in November's presidential election, are attending a ceremony at Ground Zero in New York to lay wreathes in honour of the victims.
In a joint statement, the two men vowed to come together "as Americans" and suspend their political campaigns for 24 hours.
Mr McCain earlier attended a ceremony at Shanksville, Pennsylvania, where he paid tribute to the bravery of the United Flight 93 passengers who took on the hijackers.
He said: "The only means we possess to thank them is to try to be as good an American as they were. We might fall well short of their standard, but there is honour in the effort."
Mr Obama said in a statement "the terrorists responsible for 9/11 are still at large, and must be brought to justice," in a reference to the hunt for the al-Qaeda leader Osama Bin Laden.
The presidential rivals' joint appearance is to be followed by another in the evening at a Columbia University forum to discuss their views on public service.
For Mr Bush, however, it is the last time he marks the anniversary as president.
"The president thinks about 9/11 every single day when he wakes up and before he goes to bed," White House press secretary Dana Perino said on the eve of the anniversary.
Seven years after the attacks which shocked the world, Ground Zero is a construction site.
After years of delays and disagreements over how to commemorate the dead, work has finally begun on a memorial and a new skyscraper - the Freedom Tower - which is due to be completed by 2012.