New York: A jittery, embattled United States comes together in grief on Sunday`s 10th anniversary of the 9/11 attacks which killed almost 3,000 people and plunged it into an era of war and bitter internal division.
President Barack Obama and his predecessor George W Bush will attend ceremonies at the site of the destroyed Twin Towers in New York, with Obama also flying to 9/11`s other crash sites in Pennsylvania and at the Pentagon.
With federal officials warning of a new terrorism scare, security in major cities was extraordinarily tight, and Obama has called for a "heightened state of vigilance and preparedness”.
Heavily-armed police squads and bomb sniffing dogs deployed across New York, while motorists in some neighbourhoods were forced to go through checkpoints.
As every year since the horrific events of September 11, 2001, remembrance ceremonies will centre on Ground Zero, where 2,753 of the day`s 2,977 victims died in the inferno of the collapsing skyscrapers.
But unlike previous years, the ritual of reading the names of the dead will take place against a backdrop of the gleaming, three-quarter built 1 World Trade Centre tower, rather than a chaotic-looking construction site.
Sunday will also see 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 written in bronze around the edges.
Even as US intelligence agencies chased down what officials said was a credible but unconfirmed threat of an al Qaeda attack around 9/11, Obama assured terrorism would never win.
"We will protect the country we love and pass it safer, stronger and more prosperous to the next generation," he added. "Today, America is strong and al Qaeda is on the path to defeat."
Obama and Bush will attend the ceremony together for the first time, along with victims` family members, New York Mayor Michael Bloomberg and his predecessor Rudolph Giuliani -- who led the city 10 years ago.
The 9/11 remembrances unite Americans like almost no other event. According to a poll last week, 97 percent of people remember where they were when they heard the news, on a par with John F Kennedy`s assassination.
The country was also thrilled -- with young people spilling onto the streets in Washington and New York -- at the news in May that US Navy SEALS had flown into Pakistan and shot dead al Qaeda founder Osama bin Laden.
Yet while al Qaeda is severely weakened and New York is recovering, the anniversary still finds a nation struggling to overcome the longer-term impacts of the last decade.
In Afghanistan where US forces will also hold ceremonies on Sunday at the Bagram air base with a similar event at the US embassy in Kabul, troops are stuck in a seemingly unwinnable war against a Taliban guerrilla movement few Americans understand.
Though US troops have a reduced presence in Iraq, their occupation of the country, years of vicious inter-Iraqi violence, and a host of torture scandals, have bled the US economy and sullied Washington`s image abroad.
And as unemployment and next year`s Presidential Election become the focus for most Americans, those already distant wars can seem a world away.
Leading politicians may make patriotism part of every stump speech, but the loss of more than 6,200 US soldiers in wars launched by Bush, the hundreds of slain allied troops, and the deaths of tens of thousands of Afghan, Pakistani and Iraqi civilians, rarely get a mention.
In this time of rancour, Sunday is at least a chance for brief reflection, and many ceremonies and services were being held around the world from New Zealand to Russia and Paris to honour those killed on 9/11.
British Foreign Secretary William Hague said on Sunday that al Qaeda had been weakened since the attacks.
"Al Qaeda is now weaker than at any time in the decade since 9/11 -- and political progress through peaceful protest in the Middle East and North Africa has shown it to be increasingly irrelevant to the future."
But the Taliban hit back Saturday saying the post-9/11 invasion of Afghanistan by the United States and its allies "will remain a permanent stigma on the face of Western democracy”.
In addition to the carefully stage-managed Ground Zero ceremony, Obama was to pay respects at the Pentagon, and in Shanksville, Pennsylvania where one of the four hijacked planes fell into a field, apparently after passengers overpowered the assailants.
On Saturday, a national memorial to the 40 passengers and crew of United Airlines Flight 93 who died in the Pennsylvania crash was inaugurated in a solemn dedication.
Bush praised the heorism of the passengers who battled to overcome the hijackers, saying: "One of the lessons of 9/11 is that evil is real -- and so is courage”.