Much of Sunday's memorial service in New York has been marked by silence and reflection, as America remembers the victims of the September 11, 2001 terrorist attacks.

Family members of those who died are among those attending the day-long service, being the first to tour the 26-hectare site on the original World Trade Center location.

A somber quiet blanketed the site as readers called out the names of the nearly 3,000 people who lost their lives in the attacks, including their own family members.

Wearing blue ribbons in remembrance of the victims, many readers held back tears as they said the names, telling their fathers, mothers and other fallen family members that they miss and love them. Many looked up to the sky, their faces warmed by the sunny weather, recalling the bright blue skies that began that horrible day 10 years ago.

Those in the crowd were equally moved, wiping away tears and clinging to family and friends. Family members say they felt the tranquility of trees and waterfalls set within the footprints of where the twin towers once stood.

The moments of silence marking significant moments of the day have been equally somber, including those that recalled the times when the planes hit the North and South Towers. The thousands of people gathered at Ground Zero fell silent in tribute.

French-born American cellist Yo-Yo Ma, who also performed on the one-year anniversary of September 11, played a solo during the ceremony while the Honor Guard opened the family entrance of the Memorial Plaza.

Many people paying respect to loved ones traced their hands and made rubbings over the dark granite memorial where the names of the fallen are now inscribed.

Some mourners placed American flags, photographs of the victims, flowers and other memorabilia on the granite. One firefighter put his hat on the stone, recalling the hundreds of firefighters who lost their lives trying to save those trapped in the towers. Other people were seen slumped over the memorial, covering their faces as they mourned as privately as possible.

Somber crowds also gathered in Shanksville, Pennsylvania, to remember the 40 victims killed when a hijacked plane crashed into an open field a decade ago. A ceremony also is being held at the Pentagon outside Washington, where 184 were killed when another of the hijacked planes crashed into the west side of the building.

At the Pentagon, hundreds of gatherers sat in silence as members of the military laid bouquets of white flowers at the memorial benches erected in tribute to the victims, whose ages ranged from three to 71 years.