Obama and his rival for the presidency, Republican John McCain, were to appear together twice, although briefly each time and mostly without public words. They also agreed to suspend all TV ads critical of each other.
The 2001 attacks transformed the nation in many ways, and one is that every anniversary since has found those holding or seeking office struggling for ways to appropriately pay homage. But it remained to be seen whether the McCain and Obama camps would actually refrain from sharp-edged campaigning, something hard to halt in an age of the Internet and 24-hour television news.
Obama issued a morning statement on the anniversary.
"On 9/11, Americans across our great country came together to stand with the families of the victims, to donate blood, to give to charity, and to say a prayer for our country. Let us renew that spirit of service and that sense of common purpose," he said.
But he also included a subtle dig - though at President Bush, not McCain - by including a reference to the work Bush is leaving unfinished. "Let us remember that the terrorists responsible for 9/11 are still at large, and must be brought to justice," Obama said.
McCain was to speak briefly at a ceremony near the Shanksville crash site, alongside other dignitaries and relatives of the 40 passengers and crew who were killed there. Investigators believe passengers rushed the cockpit of United Airlines Flight 93 to thwart terrorists' plans to use that plane as a weapon like the others.
In the afternoon, in New York, Obama and McCain were to visit ground zero together for a somber, silent wreath-laying in the pit that marks the largest loss of life in the attacks.
That appearance was to be followed by another in the evening at a Columbia University forum. McCain and Obama were discussing their views on public service with journalist moderators, sharing only a handshake in between their separate sessions.
Obama's only other planned outing Thursday was lunch in New York with former President Clinton.
Obama's running mate, Joe Biden, was going to an American Legion post in suburban Cleveland with an invitation-only gathering of area police, firefighters and other first responders. The Republican vice presidential candidate, Sarah Palin, was in her home state of Alaska, attending an Army ceremony to send her eldest son, Track, off to duty in Iraq and taking interviews with ABC News.
Obama and McCain last appeared together in August when they shook hands at minister Rick Warren's megachurch in Orange County, Calif., where they spoke separately about faith and values. In June they attended the funeral of NBC newsman Tim Russert, sitting next to each other at the family's request.