Tears flowed and bells tolled at Ground Zero on Sunday as Americans marked the 10th anniversary of the 9/11 attacks in a rare moment of unity for a country still shaken by the horrific attacks.

President Barack Obama and his predecessor and political foe George W. Bush stood together in New York for the main ceremony at the site of the destroyed Twin Towers. Obama then left to pay homage at 9/11's other crash sites in Pennsylvania and at the Pentagon.

At Ground Zero, the Stars and Stripes flew from cranes used in the massive project to rebuild the World Trade Center, while below, relatives of 9/11's 2,977 victims touched the names of their loves ones inscribed in bronze around a new memorial.

With federal officials warning of a new terrorism scare, lower Manhattan was under police lockdown. Security in other major cities was also tight, as Obama called for a ‘heightened state of vigilance and preparedness’.

6 MINUTES OF SILENCE FOR THE VICTIMS

The ceremony began in New York with a procession of bagpipers and singing of the national anthem, before pausing for the first of six moments of silence marking the times when the four hijacked airplanes hit their targets and the Twin Towers collapsed.

The sky over the Big Apple was initially clear, recalling the brilliant backdrop to the horrific surprise attack on the World Trade Center, where 2,753 of the day's victims, including 343 firefighters, died in the inferno of collapsing skyscrapers.

As every year since September 11, 2001, relatives of the dead took turns reading out the names, a heartbreaking litany accompanied by virtuoso cellist Yo-Yo Ma.

3/4th OF ONE WORLD TRADE CENTER

Reflecting a growing sense that it is time to turn a corner from 9/11, the Ground Zero ritual this time was accompanied by signs of optimism.

Instead of the chaotic-looking construction site and vast pit that scarred lower Manhattan for years, the ceremony now features a gleaming, three-quarter-built One World Trade Center tower and other signs of progress.

TERRORISM WON’T WIN: OBAMA

Even as US intelligence agencies chased down what officials said was a credible but unconfirmed threat of an Al-Qaeda attack around 9/11, Obama assured terrorism would never win.

"We will protect the country we love and pass it safer, stronger and more prosperous to the next generation," he said earlier. "Today, America is strong and Al-Qaeda is on the path to defeat."

CEREMONIES ALL OVER US

Commemorations were also held at the Pentagon and in Shanksville, Pennsylvania, where one of the four hijacked planes fell into a field, after passengers overpowered the assailants.

At the Pentagon, where 184 people died when a hijacked airliner smashed into the exterior of the huge complex, the ceremony closed with an army sergeant playing "taps" and then a stirring rendition of "America the Beautiful."

At the crash site of Flight 93 in Shanksville, where 40 passengers and crew died, the president of the families' association, Gordon Felt, said that healing was not the same as forgetting.

9/11 REMEMBRANCE UNITE AMERICANS

The 9/11 remembrances unite Americans like almost no other event. According to a poll last week, 97 percent of people remember where they were when they heard the news, on a par with John F. Kennedy's assassination.

For at least a day, though, 9/11 was a time to reflect and for the world to recall the surge of sympathy for the United States 10 years ago.

British Foreign Secretary William Hague declared that Al-Qaeda ‘is now weaker than at any time in the decade since 9/11’.

But the Taliban hit back on Saturday saying the post-9/11 invasion of Afghanistan by the United States and its allies ‘will remain a permanent stigma on the face of Western democracy’.