Americans marked the seventh anniversary of the Sept. 11 attacks with moments of silence, music, readings of the names of the dead and the unveiling of a new memorial.

Hundreds gathered Thursday at tearful events at all three sites where the attacks occurred in the U.S. seven years ago, killing nearly 3,000 people.

Friends and family members drop flowers in a reflecting pool to honour those who died in the attacks on the World Trade Center seven years ago during a ceremony at the site in New York on Thursday. (Mark Lennihan/Associated Press) On Sept. 11, 2001, two hijacked planes crashed into the World Trade Center in New York City, one at the Pentagon in Arlington, Va., and a fourth in a field in Shanksville, Pa.

U.S. President George W. Bush dedicated the first permanent Sept. 11 memorial at the Pentagon in honour of the 184 people who died at that crash site.

"Seven years ago at this hour, a doomed airliner plunged from the sky, split the rock and steel of this building and changed our world forever," Bush said.

"The years that followed have seen justice delivered to evil men and battles fought in distant lands. But each day, each year on this day, our thoughts return to this place. Here we remember those who died."

The $22 million memorial includes 184 benches, each one representing a victim who died when American Airlines Flight 77 crashed into the Pentagon, killing 59 people on the plane and 125 people working inside the building. There is a small reflecting pool underneath each bench.

Memorial a reminder of American resilience: Bush

"A memorial could never replace what those of you mourning a loved one have lost. We pray that you feel some comfort amid the peace of these grounds. We pray you'll find strength in knowing that our nation will always grieve with you," Bush said.

The memorial will be a "reminder of the resilience of the American spirit," he said.

The memorial ceremony included the laying of a wreath and a reading of the names of those who died there.

At Ground Zero in Lower Manhattan in New York City, hundreds of people observed four moments of silence, at 8:46 a.m., 9:03 a.m., 9:59 a.m. and 10:29 a.m. ET, the times when the first and second hijacked planes hit the World Trade Center buildings and when the south and north towers fell.

"Today marks the seventh anniversary of the day our world was broken," Mayor Michael Bloomberg told the crowd. "It lives forever in our hearts and our history, a tragedy that unites us in a common memory and a common story … the day that began like any other and ended as none ever has."

New York Gov. David Paterson acknowledged the enormous emotional pain suffered by relatives of the victims.

"A great French-Algerian writer once asked what is to be done when we have lost a loved one that gives rise to such suffering?" Paterson said.

"He answered his own question this way: 'To know how to suffer, to know how to love, and when everything falls apart, to take everything up once more.'"

Campaigning put on hold

Democratic presidential nominee Barack Obama and Republican presidential nominee John McCain agreed to appear together to pay silent respects at Ground Zero in honour of those who died. Both candidates suspended TV ads critical of each other for the day.

Meanwhile, in Washington, Bush led the moment of silence at the White House at 8:46 a.m. ET, before he headed to Arlington for the dedication of the memorial. He was joined by his wife, Laura, and Vice-President Dick Cheney and his wife, Lynne.

Later, in Arlington, he spoke of the children, friends and other loved ones who died in the attack.

"For the families and friends of the fallen, this memorial will be a place of remembrance," Bush said. "Parents will come here to remember children who boarded Flight 77 for a field trip and never emerged from the wreckage."

"People from across our nation will come here to remember friends and loved ones who never had the chance to say goodbye."

Every Sept. 11 since the attacks, Bush has observed a moment of silence on the South Lawn of the White House in honour of the victims.

Earlier in the day in Pennsylvania, about 200 people gathered at a simple ceremony in a field where United Airlines Flight 93 went down, killing 40 people on board. Investigators believe some of the passengers and crew stormed the cockpit to prevent the hijackers from using the plane as a weapon. Everyone aboard was killed when the plane crashed.

McCain spoke at the ceremony, asking every person "to be as good an American" as those who died on the flight.

McCain said those on the flight might actually have saved his life, as some believe the insurgents wanted to slam that plane into the U.S. Capitol.

"We might fall well short of their standard, but there's honour in the effort," McCain said.

With files from the Associated Press