Commemorative events have been 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.
US President Barack Obama attended ceremonies early on Sunday in New York at the site of the World Trade Center which was destroyed when hijackers flew airliners into the twin towers.
He later visited the Pentagon, also targeted by hijackers, after laying a wreath at the Wall of Names memorial in rural Pennsylvania, where a fourth passenger jet crashed into the ground near the town of Shanksville.
In the evening, Obama made a speech at a memorial event in Washington DC, repeatedly citing US commitment to its ideals and that the decade since the 9/11 attacks had strengthened the country.
"These past 10 years tell a story of our resilience," he said, adding that the decade had proven "that America does not give in to fear", that "we hold fast to our freedoms and had "shown America's resolve to defend its citizens and our way of life".
"Two million Americans have gone to war since 9/11," he said. "Too many will never come home. Those that do carry dark memories from distant places and the legacy of fallen friends."
"Nothing can break the will of a truly united America... We have overcome slavery and civil war, bread lines and fascism, recession and riots, communism and, yes, terrorism."
Obama ended his speech: "May god bless the memory of those we lost, and may god bless the United States of America."
World Trade Center memorial
George W Bush, the former president who was in office at the time of the attacks, joined Obama in New York for a memorial ceremony at the site of the towers 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 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."
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 monument consisting of massive fountains sunk into the footprints of the former towers, with the names of the dead inscribed around the edges.
Hillary Clinton, the US secretary of state, 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".
Osama bin Laden, who is believed to have masterminded the 2001 attacks, was killed by US special forces in Pakistan in May.
"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 post-9/11 war zones to dignitaries in capitals across the globe, the world also paid tribute on Sunday.
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 those killed.
In Kabul, Ryan Crocker, the US ambassador, joined dozens of embassy staff and troops for a ceremony 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.
Germany marked the anniversary with a memorial service for the victims in Berlin led by representatives of the Christian, Jewish and Muslim faiths and attended by German President Christian Wulff.
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.
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.
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."