NEW YORK — The nation solemnly observed the fifth anniversary of the Sept. 11 attacks today with moments of silence timed to mark the World Trade Center jetliner crashes and quiet remembrances held around the country.

The 16-acre trade center site in lower Manhattan fell quiet at 8:46 a.m., five years after American Airlines Flight 11 crashed into the north tower, and 9:03 a.m., when United Flight 175 slammed into the south tower.

Family members at ground zero held up signs reading "You will always be with us" and "Never forget," and quiet sobs could be heard as the moments of silence were observed. Some victims' relatives crossed themselves and wiped away tears.

"Five years have come, and five years have gone, and still we stand together as one," Mayor Michael Bloomberg said. "We come back to this place to remember the heartbreaking anniversary — and each person who died here — those known and unknown to us, whose absence is always with us."

The sorrowful task of reading the names of the 2,749 victims of the trade center attacks fell to spouses and partners.

"My love for you is eternal," said Maria Acosta, who began the annual reading of the names, including her lost boyfriend, Paul John Gill. "And we all love you very much."

President Bush opened the day with breakfast at a historic Lower East Side firehouse, mingling with firefighters and police officers who were among the first to rush to the burning skyscrapers.

The president later stood in front of a door salvaged from a fire truck destroyed that day, a flag at half-staff above him. Bush was to visit the attack sites in Shanksville, Pa., and the Pentagon later in the day before giving a prime-time address from the Oval Office.

At ground zero, family members had begun arriving before 7 a.m., some holding bouquets of roses and framed photos of their loved ones. Others wore pins bearing pictures of the victims.

"I think it's important that people remember as years go on," said Diana Kellie, of Acaconda, Mont., whose niece and niece's fiance were killed on one of the planes. "The dead are really not dead until they're forgotten."

In Shanksville, where United Flight 93 crashed to the ground, killing 40, people gathered at a temporary memorial — a 10-foot chainlink fence covered with American flags, firefighter helmets and children's drawings.

Many of the visitors, like 15-year-old Carol Fritz, had no connection to the doomed flight.

"I didn't understand when everything happened," Carol said, crying. "My kids, my grandkids are going to ask me what happened. I wanted to tell them, tell them I was here."

Moments of silence were also observed in the American and United terminals of Logan International Airport in Boston. American Airlines Flight 11 and United Airlines Flight 175 took off from Logan before slamming into the towers.

Other remembrances were planned around the country: Firefighters in Akron, Ohio, planned to display 3,000 American flags on a 10-acre plot in a western Ohio village. In Virginia Beach, Va., firefighters and members of the public planned to form a human flag.

On Sunday, Bush marked the eve of the anniversary with somber gestures and few words: He and his wife, Laura, set wreaths in small, square reflecting pools in the pit of the trade center site, one each for where the north and south towers stood.

The Bushes had descended the long ramp from street level into ground zero accompanied by New York Gov. George Pataki, New York Mayor Michael Bloomberg and Rudolph Giuliani, hailed for his work as mayor in the months after the attack.

"It took about 30 years for this terrorism to develop," Giuliani said Monday morning on ABC's "Good Morning America" as he stood at the site. "It's going to take more than five years to deconstruct them."

"I'm kind of surprised at the progress we've made," he said. "We haven't been attacked in five years. I thought we would be. I thought for sure we would be. I thank God we haven't. But we have to prepare for it."

On Sunday afternoon, the Bushes attended a memorial service at St. Paul's Chapel just off ground zero, where George Washington once prayed and where exhausted rescuers sought refuge in 2001 while they dug through the trade center rubble.

A youth choir sang "America the Beautiful" and "My Country 'Tis of Thee," and religious leaders of several faiths offered words of comfort.

At a ceremony Sunday at 7 World Trade Center, the gleaming first office tower to rise at ground zero, Pataki honored first responders and said American freedom represents "the ultimate threat" to terrorists.

Peter Gorman, president of the New York Uniformed Fire Officers Association, took note of the day's vivid blue sky and said it reminded many of the late-summer morning of Sept. 11, 2001.

"Today is still a glorious day in the glorious city of New York, the powerful state of New York, in the United States of America," Gorman said. "New Yorkers and Americans will never bow to terrorism, thanks to the U.S. military, thanks to every first responder in this country."

The anniversary dawned on a nation unrecognizable a half-decade ago — at war in Afghanistan and Iraq, governed by a color-coded terror alert system, newly unable to carry even hair gel onto airplanes.

Bush administration officials mounted a vigorous defense Sunday of the measures they had taken to protect the country, even as the nation remains divided on the Iraq war, treatment of terror detainees and surveillance measures.

"There has not been another attack on the United States," Vice President Dick Cheney said on "Meet the Press" on NBC. "And that's not an accident."

And there was a fresh reminder of the terrorist threat: An hourlong videotape posted online Sunday showed previously unseen footage of Osama bin Laden, smiling, and other commanders apparently planning the New York and Washington attacks.

An unidentified narrator said the plot was devised not with computers and radar screens and military command centers but with "divine protection" for a brotherly atmosphere and "love for sacrificing life."