The United States is marking nine years since the 9-11 terrorist attacks on New York and Washington. The anniversary comes amid controversy over plans for an Islamic center near Ground Zero and a threat to burn the Quran.
The biggest event in New York opened with the sound of bagpipes and drums commemorating the attack when two planes hit the twin towers of the World Trade Center.
New York Mayor Michael Bloomberg spoke about the impact of the attack nine years later.
"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. No other public tragedy has cut our city so deeply. No other place is as filled with our compassion, our love and our solidarity," he said.
Relatives, joined by people working to build the Ground Zero memorial, read out loud the names of the victims of the terrorist attack. The reading of the list was punctuated by a moment of silence held at 8:46 am and again at 9:03 am, the times hijacked jetliners hit the North and South towers of the World Trade Center.
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U.S. Vice-President Joe Biden and his wife were among the dignitaries who attended the New York ceremony.
"We come not to mourn but to remember and rebuild. In the words of Henry Wadsworth Longfellow's poem 'The Builders': 'All our architects of fate working in these walls of time, some with massive deeds in great, some with ornaments of rhyme,'" he said.
Interspersed in the crowd of mourners were chants of thousands of sign-waving protesters both for and against the planned Islamic center near Ground Zero.
The debate over the Mosque pits advocates of religious freedom against critics who say putting an Islamic center so close to Ground Zero disrespects the dead.
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This woman, who was holding a sign that read, "No Mosque on Ground Zero," explains why she is opposed to the plans.
"The American children, they lost their parents," she said. "The American people, they lost their family. So we should understand the feelings of America. It's not the political issue. It's the feelings of America. So our people give a lot of blood. I know that I worked as a volunteer for the second day that that happened and I saw how it is terrible, so we American people doesn't want the mosque at Ground Zero."
Also adding to the tension was the plan by a Florida clergyman to burn copies of the Islamic holy book. Reverend Terry Jones has since backed off from his plans and arrived in New York late Friday in an attempt to meet with a leading imam to discuss the proposal for the Islamic center in hopes of having it moved farther away from the Ground Zero site.
In his Saturday radio address, President Obama alluded to the contentious atmosphere marking this anniversary. "This is a time of difficulty for our country. 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," he said.
Mr. Obama also joined the nation's top defense officials in remembering victims of the September 11 terrorist attack on the Pentagon.
The ceremony included family members of the 184 people who were killed in the 2001 attack on the sprawling Defense Department headquarters.
In western Pennsylvania, first lady Michelle Obama was joined by former first lady Laura Bush at a memorial service for the victims of Flight 93. The airliner was en route from Newark, New Jersey, to San Francisco on Sept. 11, when hijackers seized control. But passengers fought back and the hijackers responded by crashing the plane. The area is to become a national memorial, and Mrs. Obama told those gathered that she believes it will become a site of inspiration, reflection and remembrance for generations to come.