The ceremony came as Hurricane Katrina left Americans once again struggling with a catastrophe that caught the nation unprepared and left citizens dead and grieving.
Mayor Michael Bloomberg opened with words of condolence for those devastated by Katrina and the terrorist bombings in the London Underground.
"To Americans suffering in the aftermath of Hurricane Katrina, our deepest sympathies go out to you this day," Bloomberg said.
In New Orleans, New York firefighters helping with the relief effort gathered around a makeshift memorial for their fallen comrades, accepting the gift of a bell from a nearby church whose steeple was destroyed in the storm.
The ground zero ceremony paused for moments of silence at 8:46 a.m., the time at which a hijacked jetliner crashed into the north tower, at 9:03 a.m., the moment a second plane struck the south tower, at 9:59 a.m., when the south tower fell and at 10:29 a.m., when the second tower collapsed.