"Today marks the seventh anniversary of the day our world was broken," Mayor Michael Bloomberg said. "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."
The ceremony at ground zero included moments of silence at 8:46 a.m., 9:03 a.m. and 9:59 a.m. -- the times when two hijacked jets slammed into the trade center buildings and the south tower fell. One more moment of silence was to be held to mark the collapse of the north tower.
Services were also being held in Pennsylvania and at the Pentagon, where a new memorial will be dedicated.
Family members and students representing more than 90 countries that lost victims on Sept. 11 read the names of 2,751 people killed in New York, one more than last year. The city restored Sneha Philip, a woman who mysteriously vanished on Sept. 10, 2001, to its official death toll this year after a court ruled that she was likely killed at the trade center.
Among the readers was Laraine Angeline, who lost brother-in-law, Steve Pollicino. "Steve, your smiles live on with us," she said. "Our separation is temporary. Our love for you is forever."
The readers also included Elodie Coleman, who lost her husband, Keith E. Coleman, and brother-in-law, Scott Thomas Coleman. Speaking to her husband and father of their two children, she said, "There is no one I admire more. Thank you for loving us so completely. We miss you and love you infinitely."
Edward Bracken, who lost his sister, Lucy A. Fishman, said she was "murdered by coward men using their religion to say they are right and we are wrong," then added, "Pray for the men and women who sleep on the ground every night in the Middle East to keep our world safe."
Relatives of victims began arriving at dawn at ground zero, now a huge construction site. American flags were draped over silent cranes. Some family members held signs saying "We miss you," "We love you" and "You will never be forgotten."
Maureen Hunt, wearing a T-shirt with a picture of her sister, Kathleen, a 9/11 victim, said that it was comforting to be at the ceremony with so many who have lost loved ones.
"This is a place for us to meet," said Hunt, who has come each year to pay her respects. "It is not getting easier to attend these ceremonies."
McCain and Obama planned to visit the site after the ceremony concluded Thursday afternoon. The candidates agreed weeks ago to pull their campaign ads for the day and were appearing together Thursday night at a forum on volunteerism and service.
Former Mayor Rudy Giuliani spoke at the ceremony -- as he has every year in New York -- drawing applause from some in the crowd. As the names were being read, Homeland Security Secretary Michael Chertoff, New Jersey Gov. Jon Corzine and New York Gov. David Paterson walked down a ramp to lay flowers in the pit where the towers stood.
Last year's reading by Giuliani, then a Republican presidential candidate, drew protests from family members who said the city was ill-prepared for the terrorist attacks under his leadership and questioned whether he should be there while running for the White House. They had no opposition to McCain and Obama' visit this year.
At the Pentagon memorial dedication, former Defense Secretary Donald Rumsfeld mourned those who "one morning kissed their loved ones goodbye, went off to work and never came home" and the airline passengers "who in the last moments made phone calls to loved ones and prayed to the Almighty before their journey ended not far from where it began."
The 2-acre park, located at the spot where American Airlines Flight 77 crashed into the Pentagon's west wall, consists primarily of 184 cantilevered benches, each bearing a victim's name.
President Bush and first lady Laura Bush marked the anniversary during a moment of silence on the South Lawn of the White House, then went to the Pentagon where the president spoke at a service there.
In Pennsylvania, at least 200 people gathered Thursday morning at an observance in a reclaimed minefield in Shanksville where Flight 93 came down after passengers reportedly stormed the cockpit to thwart terrorists' plans to use that plane as a weapon like the others. Bells were to toll and victims' names would be read as part of the service.
McCain was also attending the memorial service in Shanksville for the 40 people killed aboard the hijacked flight.
Memorials are years away from being built in Pennsylvania and New York. As in past years, two bright blue beams of light will shine at night on the New York City skyline, in memory of the fallen towers.