|The flag that flew over the World Trade Center is seen during ceremonies marking anniversary of the attacks [Reuters]
US President Barack Obama capped a day of memorial events by declaring the post-September 11 decade of war and recession had proven America's will was unbreakable if it remained united.
"Nothing can break the will of a truly United States of America," Obama said, stressing the word united, and cast the fight against terrorism as a victory alongside some of his nation's greatest achievements.
Commemorative events were held in the US to mark the 10th anniversary of the September 11, 2001, attacks, paying tribute to the thousands who died on that day.
Obama on Sunday arrived early in New York, where hijackers flew airliners into the twin towers of the World Trade Center, to visit the Ground Zero site.
He visited the Pentagon, also targeted by hijackers, after laying a wreath at the marble Wall of Names in rural Pennsylvania, where a fourth passenger jet crashed into the ground near the town of Shanksville.
Obama is also slated to deliver a speech at a memorial event for the victims in Washington, the capital.
George W Bush, the former president who was in office 10 years ago, joined Obama in New York for a memorial ceremony at the side of the World Trade Center shortly before 8:46am local time, when the first tower was struck.
Moments of silence were observed for the times that each of the four planes crashed, and the two times when the World Trade Center towers collapsed.
Speakers at the event read out the names of the almost 3,000 people who were killed in the attacks.
"From the depth of my soul," a sobbing woman called out to her deceased husband, "we will always miss you." The ceremony ended nearly five hours later with the playing of "taps" by uniformed buglers.
Instead of the chaotic-looking construction site and vast pit that scarred lower Manhattan for years, the ceremony featured a gleaming, three-quarter-built One World Trade Center tower and other signs of progress.
Sunday also saw the dedication of a simple, but moving monument consisting of massive fountains sunk into the footprints of the former towers, with the names of the dead inscribed around the edges.
Secretary of State Hillary Clinton delivered remarks on Sunday afternoon in New York at an event with victims' families, where she expressed "particular satisfaction" at being part of the team that "made sure Bin Laden was brought to justice".
"Even though the United States, thankfully, has not experienced another attack since 9/11, London has, Madrid has, Mumbai has, Islamabad has, Jakarta, Bali, other places continue to be attacked," Clinton said.
"We want to make sure that every person understands that these violent extremists are not representing any religion. They are representing evil."
From US troops stationed in dusty post-9/11 battle zones to dignitaries in capitals across the globe, the world paid tribute on Sunday.
In the first of the global memorials, the US rugby team attended an emotional service in New Zealand hours ahead of their opening World Cup match against Ireland.
With the war sparked by the attacks still raging in Afghanistan 10 years on, US soldiers paused at Bagram and other bases across the country in remembrance of the nearly 3,000 people killed when the Twin Towers came down.
In Kabul, US Ambassador Ryan Crocker joined dozens of embassy staff and troops for a ceremony at the embassy which included the lowering of the American flag, prayers and speeches.
The US embassy in Baghdad, in the Iraqi capital's heavily-fortified Green Zone, observed a minute's silence at 9:11 am local time (611 GMT).
Germany marked the anniversary with an ecumenical memorial service for the victims at Berlin's American Church led by representatives of the Christian, Jewish and Muslim faiths and attended by President Christian Wulff.
The guests, who also included former chancellor Gerhard Schroeder, leader of Germany at the time of the attacks, observed a minute of silence.
At Checkpoint Charlie, the former Cold War border crossing between East Berlin and the American sector of the city, a US flag was flying at half-mast and a sign read: "We will not forget September 11, 2001. We Berliners."
The families of British 9/11 victims attended a packed memorial service at St Paul's Cathedral in London to commemorate the 67 British citizens who died in the 2001 attacks.
And in a moment of discord, around 50 protesters brandished anti-US banners and chanted slogans outside the US embassy in London where a minute's silence was being held.
'Serving the community'
On the eve of the anniversary, US President Obama visited a soup kitchen in Washington, where he served trays of rice and gumbo to some of the city's poorest citizens.
Obama said projects to serve the community were "part of what the spirit of remembering 9/11 is all about; the country being unified and looking out for one another".
Earlier, in an internet address, Obama said the US had grown stronger since the attacks: "Thanks to the tireless efforts of our military personnel and our intelligence, law enforcement and homeland security professionals, there should be no doubt: today, America is stronger and al-Qaeda is on the path to defeat."
Meanwhile, in Shanksville, Joe Biden, the vice-president, and former presidents Bush and Bill Clinton, were present at a sombre ceremony on Saturday honouring the passengers and crew of hijacked Flight 93.
They were accompanied by relatives of the 40 victims of the crash. Speakers at the dedication of the $62m Flight 93 National Memorial also included Gordon Felt, the president of the Families of Flight 93 group.
Felt's brother Edward was on board the flight, and telephoned a friend from on board to report that the flight had been hijacked.
A white stone wall, bearing the names of those who struggled with the hijackers on the Boeing 757 airliner during the attack, was unveiled in the rural Pennsylvania field where the plane crashed.
Praising those passengers, former President Clinton said at the ceremony that they "gave the country an incalculable gift" when they saved many lives by foiling the "terrorist attack".
US authorities were vigilant across New York and Washington in response to a "credible, but unconfirmed threat" of an al-Qaeda plot to attack the cities.
"Security has been tightened across the city, in line with existing plans to do so for the anniversary, but also in response to the terror threats that US intelligence officials had earlier identified," said Al Jazeera's Asad Hashim, reporting from New York.
"In the city's financial district, where the memorial ceremony is currently ongoing, the public's movement has been severely restricted ... Only family members of victims of the WTC attacks are currently being given access to the site."