The firefighters and first responders who helped rescue New Yorkers on Sept. 11 - and later, recovered the dead - were to read victims' names for the first time Tuesday at the sixth anniversary ceremony.
Many of the first responders have become sick, or died themselves, of respiratory problems and cancers they blame on exposure to World Trade Center dust.
For the first time this year, a victim who did not die at the trade center will be recognized. The city added the name of an attorney who died of lung disease five months later to its official victims' list this year.
"That vision of the clouds over Washington and New York are still haunting our psyche, and we are living in the era of homeland security in the United States right now," presidential historian Douglas Brinkley told CBS News.
Firefighters will share the stage with former mayor Rudy Giuliani, who many victims' families and firefighters said should not speak here this year because he is running for president. Democratic Sen. Hillary Rodham Clinton, seeking her own party's presidential nomination, also planned to attend ceremonies at ground zero.
President Bush was spending the day in Washington, attending a private 7:30 a.m. prayer service at St. John's Episcopal Church and holding a moment of silence on the South Lawn of the White House.
Tributes were planned in Shanksville, Pa., where 40 passengers and crew members were killed when Flight 93 plowed into a field there, as well as in Washington, where 184 people lost their lives when a plane flew into the Pentagon.
At the main U.S. base at Afghanistan, a memorial ceremony was set to be held to coincide with the time the first hijacked airplane slammed into the twin towers.
On Capitol Hill, 9/11 Commission co-chairs Tom Kean and Lee Hamilton will be holding a news conference on the government's progress in implementing their recommendations. Also Tuesday, a House committee will be hearing from FEMA administrator David Paulison on plans for responding to emergencies.
Those who cheered the attacks are also marking the anniversary. A- one which has not been previously released, featuring both bin Laden and one of the Sept. 11 hijackers - has apparently turned up. A still photo from the video shows hijacker Walid al Shehri, wearing a white turban, superimposed over an image of the World Trade Center in flames.
As in past years, New York will hold four moments of silence to mark the times that two hijacked jetliners hit, and for the two times that each tower collapsed. For the first time since 2001, Sept. 11 is on a Tuesday, the day of the attacks.
At night, two massive beams of light will project into the sky above ground zero, reports CBS News Early Show national correspondent Jeff Glor.
In addition to first responders, others to read victims' names include city workers who participated in the cleanup, construction workers and medical examiner's officials who recovered remains and volunteers who helped serve meals at a church across from ground zero.
The ceremony was moved off the trade center site this year because of increased construction at ground zero. The victims will be remembered in a nearby park just southeast of the site, while thousands of family members planned to descend briefly below street level to lay flowers at a spot near the fallen twin towers.
After threats by family members to boycott the ceremony and hold their own shadow remembrance, the city relented and agreed to let them briefly into the pit to lay flowers on the dusty bedrock.
Giuliani, who has made his performance in the months after the 2001 terrorist attacks the cornerstone of his campaign, said last week that his appearance wasn't intended to be political.
"I was there when it happened and I've been there every year since then. If I didn't, it would be extremely unusual. As a personal matter, I wouldn't be able to live with myself," Giuliani said Friday.
Several family members worried that Zuccotti Park, just southeast of ground zero, would be too small to accommodate the thousands of people. City officials said there was actually more space available than at the previous location, and that fewer people have attending the ceremony each year.
Some New Jersey communities that lost many people in the attacks said their ceremonies were being scaled back, and a local television station, WABC-TV, initially decided not to air the four-hour-plus ceremony live. Station officials reversed themselves when viewers complained.
Mail still arrives addressed to the World Trade Center, about 200 pieces a day, reports CBSNews.com's Lloyd de Vries. There's less each year, and chances are it's coming from companies that still haven't updating their mailing lists. Most of it is returned to the mailers, although some former tenants pay a fee to have the incoming mail held and then pick it up.
The total number of victims killed six years ago - 2,974 - includes 2,750 at the World Trade Center site. Forty were killed in Pennsylvania and 184 died at the Pentagon. Those numbers do not include the 19 hijackers.
"Six years after the event, it's still dominating our lives because it changed the whole tone and tenor of behavior," said Brinkley. "Anybody that's going to an airport or trying to get on a subway or just simply having to walk into a business building and flashes their ID are in a way victims of what happened during 9/11."