A memorial service to mark the 11 September 2001 attacks on the US has been held at the former site of New York's World Trade Center.
The names of the 2,749 victims of the Twin Towers attack were read out by their brothers and sisters.
The ceremony paused for a minute's silence at 0846 (1346 GMT) when the first plane hit the north tower.
In New Orleans, New York police helping with the aftermath of Hurricane Katrina held their own ceremony.
After a memorial service held in the disaster zone, the officers - all volunteers - will return to patrolling the French Quarter and other districts of New Orleans.
'Spirit of 9/11'
President George W Bush has urged Americans to remember the spirit of 9/11 and pull together after the devastation wrought by Hurricane Katrina.
Four years on and the US is confronting another tragedy
On Sunday, the president stood in silence on the lawn of the White House with his wife Laura, Vice-President Dick Cheney and his wife Lynne at 0846 GMT during a simple ceremony held on each anniversary of the attack.
"Today, again, we are a city that meets in sadness," Mayor Michael Bloomberg said at the start of the ceremony in New York under brilliant blue skies reminiscent of the day of the attacks.
The Brooklyn Youth Chorus sang the Star Spangled Banner at the start of the ceremony.
Siblings then started reading out the list of those who died in the attacks, beginning with Gordon M Aamoth Jr, an investment bank employee.
Among those making their respects were Cynthia, John, Jane and Neil Olson, who lost their 31-year-old firefighter brother, Jeffrey James Olson.
"We feel that our family died that day," said their mother, Carol Olson of Staten Island. "He's always missing - on every birthday and every holiday."
Many of the relatives were overcome with emotion as they read out the names of the dead.
"You're still our hero, please keep watching over us," Elizabeth Ahearn said about her brother, fire lieutenant Brian Ahearn.
US Secretary of State Condoleezza Rice dedicated a poem to those who perished four years ago.
George W Bush marks 0846 at the White House
Rudy Giuliani, who was mayor at the time of the attack, praised the efforts of the emergency services who died trying to evacuate the buildings.
The reading of the names of the 11 September victims was interrupted three more times to mark the south tower attack, and the collapse of both towers.
Ceremonies were also held at the Pentagon in Washington and in Pennsylvania where the other two planes crashed.
Elsewhere, Pope Benedict XVI marked the anniversary by pleading for global renunciation of hatred and asking for people to work together for justice and peace.
Commemorations also took place in the German capital Berlin and the southern Russian city of Volgograd, where more than 50,000 people formed a 40km-long human chain.