Relatives of victims killed at the World Trade Center gathered in a park in lower Manhattan for readings from dignitaries and a recitation of the names of the dead. Later Thursday, Sens. were due at ground zero to pay silent respects.
"Today marks the seventh anniversary of the day our world was broken," Mayor Michael Bloomberg said at the start of the ceremony, calling Sept. 11, 2001, a "day that began like any other and ended as none ever has."
The ritual in New York included moments of silence in the morning at 8:46, 9:03, and 9:59 and 10:29 - the times when two hijacked jets slammed into the trade center buildings and the twin towers fell.
Services were also held in Pennsylvania and at the Pentagon, where a new memorial was dedicated at a ceremony attended by President Bush. The Pentagon Memorial contains 184 benches - one for each victim that will glow with light in the night.
Among the speakers at the New York ceremony were three who were young children when their fathers went to work at the World Trade Center seven years ago and never came home. The children are now about 10, 11 and 13.
"I remember playing in the yard with him. I remember him pulling my wagon. He was strong. He always made me feel safe," said Alex Salamone, wearing the soccer jersey of his father, John. "I wish I could remember more, but we were so young when he died."
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. At the same time, family members descended to the nearby footprints of the twin towers and paid their respects to their lost loved ones.
Relatives of victims began arriving at dawn at ground zero, now a huge construction site. American flags were draped over silent cranes, and some families held signs saying "We miss you," "We love you" or "You will never be forgotten."
The ceremony included many tearful remembrances as family members reflected on the amount of time that has passed since 2001 and expressed their support for American troops.
"They took from us innocent lives in the names of their God, and it seems some people have forgotten what happened here seven years ago," said Rosaria Reneo, sister of victim Daniela R. Notaro. "Our lives are filled with pain and always will be. Thank you to all the men and women fighting for us."
The family of Sept. 11 victim Michael Diehl went to ground zero wearing white T-shirts bearing his photo and 9/11/01.
"It's still very hard for us to come here. It doesn't get any easier," said Diehl's sister-in-law, Norma Linguito. "I just wish they'd get the memorial up so we can have something, a marker, to remember everyone."
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 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.
Many families had no opposition to McCain and Obama's visit, but some questioned whether the visit was necessary. "It's probably going to be more commercial. This really should be a day for the people who lived and worked down here," said Jane Wixted, who lost her police officer son Glen Pettit on Sept. 11.
But Pettit's former colleague, Chris DeAngelo, was glad they were coming. "One of them is going to lead this nation," he said. "And for that reason, both should come here to see what happened."
At the Pentagon memorial dedication, former Defense Secretary Donald Rumsfeld mourned the airline passengers and those who "one morning kissed their loved ones goodbye, went off to work and never came home."
The Pentagon memorial is the first of three major Sept. 11 memorials to be completed. 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.
The president and first lady 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.
In Pennsylvania, several hundred people gathered to read the names of 40 victims killed 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. McCain was among those honoring the victims.
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.
The New York ceremony included one more victim name than last year. The city restored Sneha Philip, a woman who 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.
As the nation marked the seventh anniversary of 9/11, most Americans believe the U.S. is to deal with another terrorist attack against the country, according to a new CBS News poll.
In the poll, 52 percent said they thought the U.S. was adequately prepared, compared to 39 percent who said the U.S. was not prepared. This is the first time since March 2003 - right after the U.S. invasion of Iraq - that a majority held that opinion. As recently as last September, after the revelations of a failed terrorist plot in London that summer, 56 percent said the U.S. was not adequately prepared.