A day of mourning for nearly 3,000 victims of the September 11 attacks began with moments of silence and tears near Ground Zero in New York.
Memorial services took place as observers braced for protests over a mosque planned blocks away on what is usually an anniversary free of politics.
Chants of thousands of sign-waving protesters both for and against the Islamic centre were expected after an annual observance normally known for a sad litany of families reading names of loved ones lost in the 2001 terrorist attacks.
Speaking at “hallowed ground” at the Pentagon, President Barack Obama alluded to the controversy over the mosque – and a Florida pastor’s threat, later rescinded, to burn copies of the Muslim holy book.
President Obama made it clear that the US is not at war with Islam and called the al-Qaida attackers “a sorry band of men” who perverted religion.
“We will not give in to their hatred,” Mr Obama said. “As Americans, we will not or ever be at war with Islam.”
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 at Ground Zero in New York, they urged a restrained tone.
“Let today never, ever be a national holiday. Let it not be a celebration,” said Karen Carroll, who lost her brother, firefighter Thomas Kuveikis. “It’s a day to be sombre; it’s a day to reflect on all those thousands of people that died for us in the US.”
Standing before microphones, stifling sobs, some family members who read names sought to stress sentiments on all sides of the mosque argument.
Some – including Nadine DeGrange, whose uncle, Frank Wisniewski, 54, was killed - stressed that Ground Zero is hallowed.
“I come here every year because this is the only burial ground I know. And I pray to God it remains sacred,” she said.
Others sought to embrace unity and a spirit of reaching out, which is what the developers of the Islamic centre have said is their goal.
“May we share your courage as we build bridges with other people to prevent this from happening again and to preserve human dignity for all,” said Robert Ferris, saluting the dozens of building workers who joined families in reading names.
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,” Mr 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 to mark the times the hijacked jetliners hit the north and south towers of the World Trade Centre, as well as the times the towers collapsed.
President Obama and first lady Michelle Obama attended separate services at the Pentagon in Washington and a rural field in Shanksville, Pennsylvania.
The mosque debate pits advocates of religious freedom against critics who say putting an Islamic centre so close to Ground Zero disrespects the dead. While the rallies planned in New York embroiled victims’ family members in a feud over whether to play politics, a threat to burn copies of the Koran was apparently called off.
Terry Jones, the Florida pastor who made the threat, flew to New York on Friday night and appeared Saturday on NBC’s Today show. He said his church would not burn the Koran, a plan that inflamed much of the Muslim world and drew a stern rebuke from Mr Obama.
“We feel that God is telling us to stop,” he told NBC. Pressed on whether his church would ever burn the Islamic holy book, he said: “Not today, not ever. We’re not going to go back and do it. It is totally cancelled.”
He said that he flew to New York in the hopes of meeting with leaders of the Islamic centre but that no such meeting was scheduled.
Imam Feisal Abdul Rauf, leader of the planned mosque, said Friday that he was “prepared to consider meeting with anyone who is seriously committed to pursuing peace” but had no meeting planned with Mr Jones.
Lending credence to Mr Jones’ comments, a “Burn a Koran Day” banner outside his Gainesville, Florida, church was taken down.
There was no immediate reaction to Mr Jones’ comments in Afghanistan, where on Saturday shops and police checkpoints had been set on fire as thousands of people protested against the planned burning and chanted “Death to America”.
Activists in New York insisted their intentions were peaceful. More than 1,000 protesters on both sides of the issue were expected to converge at the mosque site, a former clothing store two blocks north of the trade centre site.
“It’s a rally of remembrance for tens of thousands who lost loved ones that day,” said Pamela Geller, a conservative blogger and host of the anti-mosque demonstration. “It’s not a political event, it’s a human rights event.”
In Shanksville, Michelle Obama and former first lady Laura Bush spoke to hundreds of people gathered at a memorial service for the 40 victims of Flight 93, which crashed about 60 miles south-west of Pittsburgh.
Flight 93 was en route from Newark, New Jersey, to San Francisco when hijackers seized control. But passengers fought back and the hijackers responded by crashing the plane.
Vice President Joe Biden spoke at the New York ceremony, where 2,752 people were killed when two jetliners flew into the trade centre.