President Barack Obama has paid a visit to "hallowed ground" at the Pentagon to mark the anniversary of the Sept. 11 attacks.
Meanwhile the day of mourning for nearly 3,000 terror victims began Saturday with moments of silence and tears near ground zero, and with observers bracing for protests over a mosque planned blocks away on what is usually an anniversary free of politics.
Obama said it might be natural to focus on the searing images from "that awful morning" in 2001 when al-Qaeda struck the US and so many lives "were taken so cruelly."
The president made clear in his remarks Saturday that the US is not at war with Islam. And he denigrated the al-Qaeda attackers as "a sorry band of men" who perverted religion.
Obama said their goal was to divide and demoralize Americans. But he said "we will never hand them that victory."
He called this a day of remembrance and reflection — as well as a day of unity and renewal. "We champion the rights of every American, including the right to worship as one pleases," the president said.
"This is a time of difficulty for our country," he said. "And it is often in such moments that some try to stoke bitterness — to divide us based on our differences, to blind us to what we have in common.
But he added, "we do not allow ourselves to be defined by fear, but by the hopes we have for our families, for our nation, and for a brighter future."
In New York, chants of thousands of sign-waving protesters both for and against a planned Islamic center were expected after — and perhaps during — an annual observance normally known for a sad litany of families reading names of loved ones lost in the 2001 terrorist attacks.
Family members gathering at observances in New York and Pennsylvania brought flowers, pictures of loved ones and American flags, but no signs of opposition or support for the mosque. Reading victims' names along with rebuilders at ground zero in New York, they urged a restrained tone.
Bagpipes and drums played to open the ceremony, followed by brief comments by Mayor Michael Bloomberg.
"Once again we meet to commemorate the day we have come to call 9/11. We have returned to this sacred site to join our hearts together, the names of those we loved and lost," Bloomberg said. "No other public tragedy has cut our city so deeply. No other place is as filled with our compassion, our love and our solidarity."
Moments of silence were held at 8:46 a.m. and 9:03 a.m., the times hijacked jetliners hit the north and south towers of the World Trade Center.