Washington/New York - US President George W Bush dedicated a memorial to the victims at the Pentagon of the September 11 terrorist attacks on Thursday as Americans marked the seventh anniversary of the day that shocked the world.

"Here, we remember those who died," an emotional Bush said in Washington. "And here, on this solemn anniversary, we dedicate a memorial that will enshrine their memory for all time."

The privately-funded memorial at the Pentagon is the first to have been built since the attacks that killed 2,975 people in Washington, New York and Pennsylvania.

The vivid memorial consists of 184 benches jutting over small, individual memorial pools. Each bench is inscribed with the name of an individual killed at the Pentagon or while riding in the plane that struck the massive building on that sunny September morning.

"The years that followed have seen justice delivered to evil men and battles fought in distant lands," Bush said. "But each year on this day our thoughts return to this place."

In New York, Mayor Michael Bloomberg presided over ceremonies to remember the 2,751 people who died when two airliners struck the World Trade Centre and collapsed the twin towers.

Thousands of of people gathered at the site in New York now known as "Ground Zero" to mark the horrific events that began when the first plane hit at 8:46 am. That time was marked by a moment of silence Thursday morning amid the cloudy skies and chilly winds that contributed to the somber ceremony.

"It lived forever in our hearts and our history, a tragedy that united us in a common memory and a common story," Bloomberg said on a ramp that leads down to the deep ground that was once the site of two of the world's tallest buildings at 110 storeys.

"We return this morning as New Yorkers, Americans and world citizens, remembering the innocent people from 95 nations and territories that lost their lives together that day," Bloomberg said.

Bloomberg paid tribute to the hundreds of firefighters and police officers who rushed to the scene, only to die when the buildings collapsed to the ground amid a plume of dust and debris.

The reading of names of those who died began at Ground Zero after Bloomberg's speech. A total of 105 pairs chosen from the list of victims' families took turns reading names of all the dead.

The ceremony at the Pentagon got underway as a bagpiper walked through the memorial benches playing "Amazing Grace" as families, officials and others guests made their way into the site through tight security. One woman wiped away tears as she went through gates.

"A memorial could never replace what those of you mourning have lost," Bush said.

Bush honoured members of the US military for taking the fight to terrorists and for ensuring that the fatal day hasn't been followed by another attack on the United States.

"Thanks to the brave men and women and all those who work to keep us safe, there has not been another attack on our soil in 2,557 days," Bush said.

Former defence secretary Donald Rumsfeld, who resigned after the 2006 congressional elections, attended the ceremony. Rumsfeld was remembered for rushing to the area of the Pentagon hit by the plane to help carry victims.

"This memorial tells the story of their last terrible moments on this Earth, moments when families were destroyed, when a symbol of America's strengthen was scarred and when our country became, in the words of an American poet, acquainted with the night," Rumsfeld said.

Rumsfeld said the dedication will reinvigorate US determination to fight terrorism and preserve the country's freedom.

"In the flag that flies here above this memorial we will be reminded of what (the victims) had in common," Rumsfeld. "They fell side by side as Americans. And make no mistake, it was because they were Americans that they were killed here in this place."

Nineteen terrorist hijackers took control of two American Airlines and two United Airlines flights. One of the planes crashed in Shanksville, Pennsylvania after the passengers, having learned of the attacks in New York and Washington, stormed the cockpit. The plane crashed in a field and all 40 passengers were killed.

The Pentagon dedication was the first memorial completed on the three sites. Memorials at the World Trade Centre and in Shanksville are expected to be completed by 2011.

Senator John McCain, the Republican presidential nominee, and his Democratic opponent, Senator Barack Obama, plan to speak at Ground Zero later on Thursday. They agreed to suspend campaigning and campaign advertising for the day out of respect for the victims.