U.S. President Barack Obama was at the site where the twin towers of the World Trade Center once stood on Sunday morning to remember Sept. 11, 2001, reading from Psalms 46 at the 10th anniversary ceremony.
"God is our refuge and strength," the psalm said. "He dwells in his city, does marvellous things and says, 'Be still and know that I am God.'"
Obama was joined by other dignitaries, including former president George W. Bush, and the family members of some of the 2,983 people who perished when al-Qaida terrorists hijacked fuel-engorged jetliners and flew them into the towers, the Pentagon in Washington and a farmer's field in Pennsylvania.
At precisely 8:46 a.m. ET _ when the first plane slammed into the north tower of the World Trade Center a decade ago _ a moment of silence was observed at Ground Zero, now the site of a new memorial with roaring waterfalls cascading into acre-sized caverns where the two towers stood.
There was another moment of silence observed 16 minutes later, when the second plane smashed into the south tower.
A few blocks away, Prime Minister Stephen Harper paid tribute in an afternoon ceremony to the 24 Canadians who died and will now be honoured at the British Garden at Hanover Square in lower Manhattan. Britain's 67 victims are already honoured there; now Canada's and Australia's will be too as the sliver of park space is renamed the Queen Elizabeth II Garden.
Describing it as a "gracious gesture" by British officials, Harper called it an "honour to accept the offer to include in this beautiful place an official commemoration of the Canadians whose lives were taken so cruelly 10 years ago today."
"In the shadow of the evil of September 11, 2001, we must not forget our capacity for goodness, and our knowledge of what is right, which is written in the hearts of all men," said Harper.
"Yes, on September 11, 2001, lives were taken, in an act of heartlessness beyond words. But in response, lives were given — freely, nobly, in acts of courage beyond compare."
The family members of Canada's victims, and a collection of Canadian police officers and rescue workers, were also on hand for the ceremony.
The 9-11 anniversary dawned with both New York and Washington, D.C., under heightened security amid revelations that both cities were targets of new, credible terrorist threats. In Manhattan, police officers and bomb-sniffing dogs swarmed the streets; vehicle checkpoints were set up every few city blocks.
The newest threats served to underscore that even 10 years later, the United States has been forever changed by the attacks. The ensuing war on terror launched by Bush in Iraq and Afghanistan has killed more than 6,000 American soldiers, and all but bankrupted the country as it struggles to cut its $14 trillion national debt.
At Ground Zero on Sunday morning, initially sunny skies gave way to leaden clouds as the ceremony's haunting centrepiece began: the reading of the victims' names by family members, many of them children or young teens. Their recital of the names left many in the crowd emotional, including former first lady Laura Bush, who wiped her eyes repeatedly.
Nicholas Gorki remembered his father, "who I never met because I was in my mother's belly."
"I love you, father," he said. "You gave me the gift of life, and I wish you could be here to enjoy it with me."
New York Mayor Michael Bloomberg paid special tribute to the children whose family members died on 9-11.
"Although we can never un-see what happened here, we can also see that children who lost their parents have grown into young adults," he said. "Good works have taken root in public service."
Bush also delivered a reading, this one a portion of a letter written by Abraham Lincoln to a mother who lost five sons in the U.S. Civil War.
The former president was greeted by a smattering of applause from the crowd. Bush famously visited Ground Zero shortly after the attacks in 2001, paying tribute to firefighters and rescue workers as they continued to root through smoking mountains of twisted metal looking for survivors.
Before the ceremony got under way, Obama and Bush held hands with their respective wives as they examined a wall etched with the names of the dead at the new memorial, a stand of white oak trees nearby. The memorial opens to the general public on Monday.
It sits next to a large-scale construction project that includes office towers, a transportation hub and a cultural centre. The complex's signature skyscraper, One World Trade Center, is under construction and will be the tallest building in the U.S. once it's completed.
The Obamas then travelled to Shanksville, Pa., for a memorial ceremony paying tribute to the passengers who died in the crash of United Flight 93 after fighting back against hijackers. They placed a wreath at the Flight 93 National Memorial.
They also attended a wreath-laying ceremony at the Pentagon, where hijacked American Airlines flight 77 crashed killing everyone onboard and 125 people on the ground.
Obama was to wrap up the day with remarks on Sunday night at a "Concert for Hope" at the Kennedy Center in Washington.
Around the world, ceremonies were held to remember 9-11 and its painful aftermath, including in Canada.
Memorial events were held in Ottawa, Edmonton, Toronto, Calgary and the tiny Newfoundland town of Gander, where thousands of stranded travellers were given shelter and comfort by the community's residents.
U.S. Ambassador David Jacobson and Premier Kathy Dunderdale were on hand in Gander, which was recognized last week in Washington, D.C., with an international resiliency award.
Obama sent a letter to Harper last week thanking Canadians for their help, saying Canada showed itself to be a true friend during one of the darkest moments in U.S. history. He paid special tribute to Gander.
A day earlier, Obama visited Arlington cemetery, volunteered at a D.C. food bank and urged Americans to devote more time to public service. He also conducted a national security meeting about the ongoing terrorist threats to the United States.
The president directed his administration "to pursue vigorously all threat information and to ensure a heightened state of vigilance and preparedness as we commemorate the anniversary of the 9/11 attacks," said a White House readout of the meeting.