Family members of the victims of the 9/11 attacks gather at Ground Zero before the start of ceremonies marking the 10th anniversary of the 9/11 attacks on the World Trade Center, in New York Sept 11, 2011. [Photo/Agencies]

Sunday's Ground Zero ceremony was set to include moments of silence marking when planes hit the twin towers as well as when they collapsed. Other moments of silence will mark when a plane hit the Pentagon and another crashed in Shanksville after passengers fought back against the hijackers.

Bush, who has kept a low profile since leaving office, was in Shanksville on Saturday. "The memory of that morning is fresh, and so is the pain," Bush told a crowd at the site.

'Their lives mattered'

New Jersey Governor Chris Christie spoke on Saturday at the opening of a monument to the 746 residents of his state killed in the attacks. The "Empty Sky" memorial in Liberty State Park, across the Hudson River from the World Trade Center, has the names of the dead etched on two 30-foot (9.1 meters) tall walls, each 208 feet and 10 inches (63.7 meters) long - the exact width of the twin towers.

"Their lives mattered," Christie said at the ceremony, which began late because security slowed traffic. "That's why we built this memorial and that's why we come here today."

Security concerns were high in Washington, too. Authorities shut down part of Dulles International Airport in northern Virginia outside the US capital on Saturday because of a suspicious object but later said no explosives were found.

New Yorkers, accustomed to heightened security and alerts that have become commonplace over the past decade, appeared to take the increased police presence in stride.

A decade later, after a faltering start, there are signs of rebuilding progress at the World Trade Center site. The new One World Trade Center skyscraper rises more than 80 stories above the ground as it inches to its planned 1,776 foot (541 meters) height - symbolic of the year of America's independence from Britain.

The memorial plaza is ready and the neighborhood has enjoyed a revival, making it a trendy Manhattan place to live.

The 2001 attacks were followed by US-led wars in Afghanistan and Iraq, the latter of which Obama opposed. The United States still has thousands of troops deployed in both countries.

But the weak US economy has become the biggest concern for many Americans. Obama said, "After a hard decade of war, it is time for nation building here at home."