Ceremonies have been taking place in the US to mark the third anniversary of the 11 September 2001 attacks in which about 3,000 people died.
Many relatives carried photographs of their loved ones
In New York, relatives and officials gathered for a public reading of the names of those killed when two hijacked planes hit the World Trade Center.
The moment of impact and the fall of its twin towers were marked by silence.
In Washington, a commemoration of victims of the Pentagon attack was attended by the US defence secretary.
Nineteen al-Qaeda Islamist militants hijacked four US domestic airliners on 11 September 2001. They flew two into the Twin Towers of the World Trade Center, one into the Pentagon and the fourth crashed short of its target.
President George W Bush - in a rare live radio address to the nation on Saturday - pledged there would be no let-up in his war on terror.
"The United States is determined to stay on the offensive and to
pursue the terrorists wherever they train or sleep or attempt to set
down roots," he said.
In New York, bagpipes and drums sounded as events got under way at Ground Zero, and church bells could be heard tolling in the distance before silence fell at
0846 (1246GMT) - the exact time the first plane struck one of the Twin Towers.
THIRD ANNIVERSARY EVENTS
A memorial is dedicated to Staten Island victims at the ferry terminal across the harbour from Ground Zero
Communities nationwide gather for services at fire stations, memorial
dedications, bell-ringing events and flag ceremonies
President George W Bush attends church service near the White House before observing a silence on the South Lawn
John Kerry, Mr Bush's Democratic challenger, attends a memorial in his home city, Boston
"We come here to remember and to ask the country and the
world to remember the names of those we lost three years ago,"
Mayor Michael Bloomberg told the audience.
"We will never forget that each person was
someone's son or daughter."
Three other moments of silence were observed - at 0903, 0959 and 1029 - reflecting the second impact and the fall of the skyscrapers.
In what has become an anniversary tradition, the names of the 2,749 victims at Ground Zero were read out - this year, by parents and grandparents of the dead.
Some carried photos of their loved ones; the voices of others cracked with emotion as they paid brief tribute on the podium.
Last year it was children who recited the names.
Many family members wept quietly and embraced one another as the ceremony continued, with solemn music playing softly in the background.
Families were allowed to walk down into the towers' "footprints", ground once combed for the tiniest fragments of human remains. The remains of about 40% of the New York victims have still not been identified.
At sundown, light beams representing the towers will be projected into the sky and remain on through the night.
At the Pentagon in Washington, where 184 people died, officials - including Defence Secretary Donald Rumsfeld - and relatives laid wreaths and observed a minute's silence.
Bells were to toll across Pennsylvania, where the fourth plane came down with the loss of 40 lives.
At a ceremony on 4 July, the cornerstone of the future 541-metre (1,776-foot) Freedom Tower was laid on the Ground Zero site.
is due to be completed by 2009.