Americans on Sunday remembered the horror of September 11, 2001, and the nearly 3,000 people who died in the hijacked plane attacks as authorities worked to ensure the emotional 10th anniversary was peaceful.

Law enforcement authorities in New York and Washington were on high alert against what was described as a 'credible but unconfirmed' threat of an al Qaeda plot to attack the US again a decade after the toppling of the World Trade Center's twin towers by hijacked airliners.

Security was especially tight in Manhattan, where police set up vehicle checks on city streets as well as bridges and tunnels coming into the city.

President Barack Obama and former President George W. Bush, who was president at the time of the attacks, were scheduled to be among the dignitaries at the Ground Zero site in New York.

They were set to join victims' families to hear the reading of the names of those who died on September 11. Bells will toll across the city.

In the attacks, 19 men from the Islamic militant group al Qaeda hijacked airliners and crashed them into the World Trade Center, the Pentagon outside Washington and a field in Shanksville, Pennsylvania.

To mark the attacks, Obama was set to visit all three sites.

'Thanks to the tireless efforts of our military personnel and our intelligence, law enforcement and homeland security professionals, there should be no doubt: today, America is stronger and al Qaeda is on the path to defeat,' Obama said in his weekly radio and Internet address.

US forces killed al Qaeda leader Osama bin Laden in Pakistan in May.

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.

New Jersey Governor Chris Christie spoke 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 tall walls, each 208 feet and 10 inches 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 ft 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.'-Reuters