PHOTOS: On 11th anniversary solemn US looks back at 9/11

Hundreds of Americans, including victims' families, gathered on Tuesday at Ground Zero in New York, the Pentagon, Pennsylvania and Washington to remember nearly 3,000 victims of the worst terrorist attacks in United States history that devastated the country 11 years ago.

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The low profile ceremonies across America was led by President Barack Obama who along with his wife Michelle and Presidential staff paused for a minute at the White House lawns in memory of the victims of the nightmarish events of September 11, 2001.

While Obama was sporting a dark suit, the first lady was in a dark purple dress, as three bells tolled. They stood with heads bowed, facing the National Mall in the distance. Most stood with their hands crossed in front of them.

From White House Obama headed to Pentagon to attend memorial ceremony.

"I've always said that our fight is with the Al Qaeda and its affiliates, not with Islam or any other religion. This country was built as a beacon of freedom and tolerance. That's what's made us strong, now and forever," Obama said at the event at the Pentagon held to mark 9/11 anniversary.

"Al Qaeda's leadership has been devastated, and Osama bin Laden will never threaten us again. Our country is safer and our people are resilient," he said.

Those attending the ceremonies across the country paused for a moment of silence to mark the time when each of the four planes hijacked by Al Qeada terrorists rammed into twin towers of World Trade Centre, into Pentagon and into a field near Pennsylvania in a co-ordinated attacks that shook the world.

Obama's Republican rival Mitt Romney also kept himself away from political events in the thick of the presidential campaign to keep the sanctity of the occasion.

Both Obama and Romney are taking down their negative advertisements in a brief truce from political battle.

Romney in a statement said that America will never forget those who perished in the terrorist attack.

"On this most somber day, those who would attack us should know that we are united, one nation under God, in our determination to stop them and to stand tall for peace and freedom at home and across the world," Romney said.

In New York City, remembrances marked one of its most tragic days.

"Today, we honour the thousands of innocent men, women, and children who were taken from us too soon eleven years ago," said Joe Daniels, president and CEO of 9/11 Memorial in New York City.

At Ground Zero -- the site of the twin towers -- family members read out the names of their loved ones who died in the attacks and held pictures as music played.

One man held high a sign that had the name Danielle Kousoulis with pictures of the young woman attached and said "We miss you" and "We love you."

There was a moment of silence at 8:46 am to mark when the first commercial plane struck the north tower.

Houses of worship across New York City rang their bells in remembrance.

A skyscraper at One World Trade Center is near completion and will again be the tallest building in New York, as were the Twin Towers before they came down.

"Eleven years ago, on a morning very much like this, terrorists attacked our symbols of our strength and took the lives of people from more than 90 countries," said Secretary of Defense Leon Panetta at the Pentagon.

Security was tightened across America, especially at New York's Ground Zero and the capital Washington DC.

The stepped up security measures came as Taliban, ahead of the anniversary, threatened "US forces face utter defeat in Afghanistan and Americans are unsafe wherever they go in the world."

Romney's vice presidential running mate Paul Ryan said like all Americans, he will never forget the moment that the country came under attack.

"For me, this is a day to remember those who perished on that day of terror, including the first responders," he said.

"It is also a day to pay tribute to all those who have worked quietly and tirelessly both on the home front and abroad to prevent a repetition of such terrible events."

House Democratic Whip Steny H Hoyer said 11 years later, Osama bin Laden is no longer a threat to the country and the Al Qaeda has been severely weakened.

US Senator John Cornyn, a member of the Senate Armed Services Committee, said, "On this somber anniversary we stand together as Americans to remember the nearly 3,000 lives that were taken on a September morning just 11 years ago."

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