The anniversary was to be incorporated into the National Memorial Day Parade in Washington, where special tributes were scheduled for the first responders to the attacks and to the relatives of the thousands killed. Actor Gary Sinise, a veterans advocate who played Lt. Dan in the film "Forrest Gump," and Medal of Honor recipients from the Korean and Vietnam wars were among the guests.
The public holiday recognizes America's war dead.
President Barack Obama was participating in a wreath-laying ceremony at the Tomb of the Unknowns at Arlington National Cemetery.
"Our nation owes a debt to its fallen heroes that we can never fully repay, but we can honor their sacrifice," Obama said at a Memorial Day service at the cemetery. "And we must."
Meanwhile, U.S. troops fighting in Afghanistan paused to remember the fallen in Memorial Day services, with some praying and holding flag-raising ceremonies to recognize the more than 1,400 who have been killed in combat there since the war began a decade ago.
"We reflect on those who have gone before us. We reflect on their service and their sacrifice on behalf of our great nation," said Brig. Gen. Lewis A. Craparotta, who commands a Marine division in Afghanistan's southern Helmand province. "We should also remember those serving today who embody that same commitment of service and sacrifice. They are committed to something greater than themselves and they muster the physical and moral courage to accomplish extraordinary feats in battle."