Americans are holding a day of prayer and remembrance to mark the second anniversary of the 11 September attacks which killed about 3,000 people.
Relatives of victims descended into the heart of Ground Zero
A hush fell across the land at 1246 GMT as people silently marked the moment two years ago when the first hijacked airliner crashed into the World Trade Center.
The ceremony at Ground Zero in New York featured children who lost parents in the attack reading out the victims' names, accompanied by personal messages of grief and a mournful flute solo.
The ceremony began with a children's choir singing the US national anthem as the stars and stripes flag was paraded, then Amazing Grace was played on bagpipes.
Mayor Michael Bloomberg said children had been chosen to lead the service because "it is in them that the spirit of our city lives on".
Ground Zero timetable
0830 (1230GMT) Ceremony begins
0846 (1246) Silence marks first plane crash
0903 (1303) Silence marks second plane crash
0959 (1359) Silence marks first tower collapse
1029 (1429) Silence marks second tower collapse
1200 (1600) Ceremony ends
1915-1945 (2315-2345) 'Tribute in Light' will be lit
In Washington, President George W Bush attended a
private church service, then stood on the White House lawn with First Lady Laura Bush and Vice President Dick Cheney for the minute's silence.
Bin Laden tape
Emotions were heightened on the eve of the anniversary by a new videotape appearing to show Osama Bin Laden alive and praising those who carried out the attacks.
The Arabic television station Al-Jazeera broadcast the videotape, accompanied by an audio recording of a speaker said to be the al-Qaeda leader and another voice said to be his deputy Ayman al-Zawahri.
CIA experts who analysed the tapes said they had "high confidence" the Zawahri voice was genuine, but their verdict on the Bin Laden voice was "inconclusive", Reuters reported.
Soon after the videotape was broadcast, President Bush warned that al-Qaeda "still plots against our people".
"The forces of global terror cannot be appeased, and they can't be ignored. They must be hunted, they must be found and they will be defeated," he said.
His words were echoed by US Defence Secretary Donald Rumsfeld, who said "the fight for freedom continues".
Speaking at Arlington National Cemetery, he said "we know if we do not fight the terrorist over there in Iraq, in
Afghanistan and across the world, then we will have to face them here".
Mr Bush remained in Washington to lead services for the 184 people killed at the Pentagon, where a memorial garden in honour of victims was being dedicated.
A service is also being held on the Pennsylvania hillside where 44 people died after passengers stormed the cockpit of Flight 93, overcoming the hijackers.
Thousands of smaller remembrance services are being held throughout the country.
Former New York mayor Rudolph Giuliani, widely praised for his response to the attacks, attended the New York ceremony.
New York's scar
As happened last year, the families of the victims were allowed to descend the seven stories below ground level into the heart of Ground Zero to lay wreaths.
It may be the last time that they are allowed to stand inside the scar in the earth.
Construction is to start soon on a new tower with offices, parks and a memorial.
A small ceremony, including a performance from a Welsh choir, is also being held at a garden in Lower Manhattan dedicated to the 67 Britons killed in the disaster.
In solidarity with their American counterparts, some 100 British policemen attended the Ground Zero ceremony in full dress uniform.
The memorial ceremony will conclude at 1200 (1600GMT), but in the evening, as dusk falls, the Tribute in Light - two intense blue beams of light representing the fallen towers - will shine in the Manhattan skyline once more.
The "Bin Laden" recording was probably made in April or early May in an "undetermined mountain area", according to the broadcaster, Qatar-based al-Jazeera.
"Whoever wants to learn loyalty and courage in support of religion should learn from [Said] al-Ghamdi, Mohammed Atta, Khaled al-Mihdhar, Ziad Jarrah, Marwan al-Shehhi and the others", the speaker said.
The other speaker, said to be Mr Zawahri, urged Iraqis to "rely on God and devour the Americans, like lions devour their prey - bury them in the Iraqi graveyard".
The BBC's security correspondent Frank Gardner says the video is important because it is the first new footage of Bin Laden seen in more than a year.
It was almost certainly shot in Afghanistan or Pakistan, he says.