Relatives of victims descended into the heart of Ground Zero
The tolling of bells and moments of silence marked the second anniversary of the 11 September attacks as Americans remembered the more than 3,000 victims amid new terror warnings.
Children led the tributes to victims at Ground Zero in New York, reading out the names of those who died in the World Trade Center, at a ceremony lasting more than three hours.
Thousands of victims' relatives - many of them clutching pictures of loved ones and weeping - heard the 200 children speak in pairs about their family losses, sometimes overcome with emotion and stumbling over pronunciations.
A hush fell across the land at 1246 GMT as people silently marked the moment when the first hijacked airliner crashed into the twin towers. There were more silences to remember the moments when the towers fell.
Spirit of New York
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".
The ceremony was relatively low-key, dominated by private family grief, with politicians largely kept in the background.
It began with a children's choir singing the national anthem and bagpipers playing Amazing Grace as the stars-and-stripes flag was paraded.
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.
"We remember the heroic deeds. We remember the compassion, the decency of our fellow citizens on that terrible day," Mr Bush said.
The anniversary was overshadowed by fresh warnings about the threat from al-Qaeda, including a new videotape appearing to show Osama Bin Laden alive and praising the hijackers.
The US State Department said it expected al-Qaeda to "strive for new attacks that will be more devastating than the 11 September attack, possibly involving non-conventional weapons such as chemical or biological agents".
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
US Defence Secretary Donald Rumsfeld attended a ceremony at Arlington National Cemetery in Virginia to mourn those who died when a third hijacked plane hit the Pentagon.
"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," he warned.
The BBC's Matt Frei in New York says some relatives feel that the Bush administration has misused the spirit of 9/11 to justify its foreign policy - notably the wars in Afghanistan and Iraq.
Some relatives are also angry at the plans to build a new tower with offices, parks and a memorial at Ground Zero - a site they regard as sacred.
Both France and Pakistan have criticised US foreign policy on the anniversary.
The French Foreign Minister, Dominique de Villepin, said the thinking of the powerful neo-conservative lobby in Washington was ill-suited to a world in which brute force produced only
Pakistan's President Pervez Musharraf told the BBC that relations were deteriorating between the Western and Islamic worlds.
New York's scar
The 9/11 victims' families were allowed to descend the seven stories below ground level into the heart of Ground Zero to lay wreaths.
Former New York Mayor Rudolph Giuliani, widely praised for his response to the attacks, attended the ceremony.
In solidarity with their American counterparts, some 100 British policemen attended the ceremony in full dress uniform.
In the evening, the Tribute in Light - two intense blue beams of light representing the fallen towers - will shine in the Manhattan skyline once more.
A service was also held on the Pennsylvania hillside where 44 people died after the fourth hijacked plane crashed - apparently after a struggle between the passengers and hijackers.
Emotions were heightened on the eve of the anniversary by the new al-Qaeda videotape, broadcast by the Arabic television station al-Jazeera.
It was accompanied by an audio recording of a speaker said to be Osama Bin Laden 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.
The speaker said to be Zawahri urged Iraqis to "rely on God and devour the Americans, like lions devour their prey - bury them in the Iraqi graveyard".