New York has paused to remember the times two planes struck the World Trade Center on 11 September 2001 - an attack that killed nearly 3,000 people.
US President George W Bush has led commemorations of the seventh anniversary of the 9/11 attacks, which left nearly 3,000 people dead.
Four moments of silence are being held to mark the times when four hijacked passenger planes hit the Twin Towers, the Pentagon and a Pennsylvania field.
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.
Presidential candidates Barack Obama and John McCain are attending a ceremony at Ground Zero in New York.
In Washington, Mr Bush dedicated a new memorial at the Department of Defense to 184 people killed there.
President Bush dedicated a new memorial at the Pentagon, where 184 died.
"The worst day in America's history saw some of the bravest acts," he said.
New York City Mayor Michael Bloomberg opened Thursday's memorial event at Ground Zero, where families of the victims read out a roll call of those who died.
"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."
Names of those who died in the attacks were read out in New YorkEnlarge Image
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".
Rivals' unity
Silences were observed at the moments each of the Twin Towers was struck and fell.
The attacks, which triggered the US-led invasions of Afghanistan and Iraq and the Bush administration's war on terror, are regarded as the defining moment of President George W Bush's time in office.
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.
At the Pentagon, he paid tribute to the acts of courage shown by Americans seven years ago, saying: "The worst day in America's history saw some of the bravest acts in America's history."
In a joint statement, the two men vowed to come together "as Americans" and suspend their political campaigns for 24 hours.
A flag was raised over the Washington memorial, which was built at a cost of $22m (£12.6m) on a 1.9-acre (0.77-hectare) parcel of land within view of the crash site.
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.
The president was joined in the US capital by Defence Secretary Robert Gates and his predecessor, Donald Rumsfeld.
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 Bush had stood earlier for a moment of silence with First Lady Laura Bush on the White House lawn at the time the first plane hit the World Trade Center.
It is the last time Mr Bush 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.
'Put aside politics'
Senators Obama and McCain, the Democratic and Republican nominees in November's election, will appear together at Ground Zero in the afternoon to lay wreathes in honour of the victims.
In a joint statement from the campaigns announcing their decision to visit Ground Zero together, the two men vowed to come together "as Americans" and suspend their political campaigns for 24 hours.
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.
"We will put aside politics and come together to renew that unity, to honour the memory of each and every American who died, and to grieve with the families and friends who lost loved ones," the statement said.
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.
Their 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 ceremony in downtown Manhattan is marking the times when the planes hit the Twin Towers, and when each tower fell - pausing for silence at 0846, 0903, 0959 and 1029.
"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.
Family members and students representing the 90 countries that lost people in the attacks also read out the names of all the 2,973 dead.
Seven years after the attacks which shocked the world, Ground Zero is a construction site.
Seven years after the attacks which shocked the world, Ground Zero is a construction site.
9/11 MEMORAIL TIMETABLE 1340BST: New York World Trade Center ceremony begins1346: Moment of silence (time first plane hit North Tower)1346: President Bush has moment of silence at White House1403: Moment of silence (time second plane struck South Tower)1430: Mr Bush in Washington for 9/11 Pentagon Memorial dedication1459: Moment of silence (time South Tower fell)1529: Moment of silence (time North Tower fell)1545: Members of Congress gather on the West Steps to honor those killed and injured on 9/11
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.
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.
On Wednesday, Mr Bloomberg called for the abolition of the WTC planning agency, saying the reconstruction was "frustratingly slow".
"Most important, the memorial must be completed by the 10th anniversary. No more excuses, no more delays," he added.
On the eve of the anniversary, a top US military commander warned new tactics were needed to win the conflict in Afghanistan, which the US and its allies invaded three months after 9/11.
They aimed to topple the Taleban and hunt down Osama Bin Laden, who the US believes masterminded the attacks.
Admiral Mike Mullen believes insurgents are launching attacks from neighbouring Pakistan, and US-led forces must target their "safe havens" in that country.
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