"Today we declare once more we will never hand them that victory ... for our cause is just, our spirit is strong and our resolve unwavering," the president said at the Defense Department's headquarters, where a hijacked plane slammed into the Pentagon, killing 184 people on Sept. 11, 2001.
"They may seek to exploit our freedoms, but we will not sacrifice the liberties we cherish or hunker down behind walls of suspicion and mistrust. They may wish to drive us apart, but we will not give in to their hatred and prejudice," Obama said, despite the terrorists' efforts to spark conflicts among faiths. "As Americans we are not - and never will be - at war with Islam."
This year's remembrances of the 2001 attacks took place with growing public suspicion of Muslims, an emotional dispute over an Islamic community center and mosque planned near ground zero in New York City, and a Florida pastor's threat to burn Qurans.
Nine Years Later: Remembering 9/11
A man prays at the reflecting pool at the World Trade Center site during a ceremony to mark the ninth anniversary of the terrorist attacks in New York on Saturday. Nearly 3,000 people died on Sept. 11, 2001, when terrorists flew hijacked planes into the Pentagon and New York's World Trade Center towers.
Nine Years Later: Remembering 9/11
Family members of the victims of 9/11 leave handwritten notes and flowers at the site where the twin towers once stood.
Nine Years Later: Remembering 9/11
A family member weeps during the reading of the names of 9/11 victims at the ceremony commemorating the terrorist attacks on the World Trade Center in New York.
Nine Years Later: Remembering 9/11
Brooklyn native Anna Sereno holds a photo of her son, Arturo Angelo Sereno, as people gather for the ceremony at Zuccotti Park, adjacent to ground zero.
Nine Years Later: Remembering 9/11
Luke Pavlenishzili, 2, riding on the shoulders of his father, George Pavlenishzili, offers a rose to New York firefighter Joe Huber during the memorial service in New York.
Nine Years Later: Remembering 9/11
Alyson Low, of Fayetteville, Ark., holds a photo of her sister Sara Low on Saturday in New York.
Nine Years Later: Remembering 9/11
A man holds his hat while praying at the Flight 93 National Memorial in Shanksville, Pa., during a service of remembrance on Saturday.
Nine Years Later: Remembering 9/11
A temporary memorial sits at the crash site of Flight 93 in Shanksville, Pa..
Nine Years Later: Remembering 9/11
President Barack Obama comforts a 9/11 family member during a wreath laying ceremony at the Pentagon Memorial on Saturday.
Nine Years Later: Remembering 9/11
Friends and family of 9/11 victims gather at ground zero in New York on Saturday.
"This is a time of difficulty for our country," Obama said before the ceremony, in his radio and Internet address. "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 on this day, we are reminded that at our best, we do not give in to this temptation," Obama said.
With the war still raging against al-Qaida and the Taliban leaders harboring them in Afghanistan, Defense Secretary Robert Gates said the anniversary of the attacks provides a chance to reflect on what they have meant "for an entire generation of young Americans who answered the call to serve."
The Rev. Terry Jones backed away from his plan to burn the Muslim holy book, saying on NBC's "Today" show Saturday that "God is telling us to stop."
In the GOP's weekly address, Sen. Jon Kyl, R-Ariz., echoed Obama's plea for a common purpose. Kyl called for the country to "recapture the unity that allowed us to come together as a nation to confront a determined enemy."
Without mentioning the president by name, Kyl seemed to question the Obama administration's commitment to the fight against terrorism begun by President George W. Bush. Obama recently declared an end to combat missions in Iraq even as he pledged to renew war-fighting efforts in Afghanistan and pursue al-Qaida terrorists.
"The fact that none of the subsequent attempts to attack us have succeeded seems to have removed some of the urgency and commitment so necessary to succeed in war," Kyl said.
Remembering September 11
The twin towers of the World Trade Center burn behind the Empire State Building in New York on Sept. 11, 2001. In a horrific sequence of destruction, terrorists slammed planes into the WTC towers, the Pentagon in suburban Virginia and the central Pennsylvania countryside. In all, more than 2,700 people died that day. This year marks the ninth anniversary of the attacks.
An amateur photographer captured United Airlines Flight 175 as it was about to strike the south tower of the World Trade Center minutes after American Airlines Flight 11 smashed into the north tower.
People in front of New York's St. Patrick's Cathedral react with horror as they look down Fifth Avenue toward the World Trade Center towers after planes crashed into their upper floors.
Rescue workers carry fatally injured New York City Fire Department chaplain Mychal Judge from the lobby of the north tower.
The south tower of the World Trade Center collapsed at 9:59 a.m., followed by the north tower, which collapsed at 10:28 a.m.
In this photo released this year by the New York City Police Department, smoke and ash engulf lower Manhattan after the collapse of the World trade Center towers.
People run away from the World Trade Center after the collapse of the towers.
Dust and debris cloud the air near the World Trade Center site.
A man stands in the rubble and calls out asking if anyone needs help after the collapse of the first tower.
Edward Fine covers his mouth as he walks through the debris after the collapse of one of the towers.
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