The events Wednesday morning included patriotic readings and songs, and the placing of flowers at ground zero by the families of the victims.
"They were our neighbors, our husbands, our children, our sisters, our brothers and our wives. They were our countrymen and our friends. They were us," Mayor Michael Bloomberg said.
As Yo-Yo Ma played the Sarabande to Bach's C minor cello suite, former Mayor Rudolph Giuliani began the solemn reading of the names of the 2,801 people missing and dead in the World Trade Center attack. The recitation ended with the sound of taps, a string quartet playing "The Star Spangled Banner," and the ringing of bells across New York City.
Thousands of spectators and participants are at the 16-acre site where once the city's tallest skyscrapers stood.
The first moment of silence occurred at 8:46 a.m. EDT, marking the instant when American Airlines Flight 11 crashed into the north tower of the gargantuan complex. A second was held at 10:29, at the time when the second jetliner hit the south tower.
The anniversary ceremony got an early start in New York City.
Overnight, bagpipers led solemn pilgrimages from each of New York's five boroughs, reports CBS News Correspondent Lou Miliano, some beginning from as far as 19 miles away.
Hundreds of spectators applauded, held American flags and ran alongside taking photographs as the procession made its way along a boulevard illuminated by the lights of emergency vehicles.
More than 400 members of the fire department, police and other agencies were killed in the largest rescue effort in U.S. history. "This solemn procession represents a call to duty that saved so many, yet cost so much," said Bloomberg.
The mournful marches stretched more than 80 miles through a mosaic of neighborhoods, past rundown tenements in the Bronx, along mansions and museums on Fifth Avenue, by glass office towers of midtown Manhattan and through the winding streets of Greenwich Village to the now-empty 16-acre trade center site.
The marchers represented the uniform services of New York, reports CBS News Correspondent Cami McCormick: The fire department, the police department, the port authority, corrections and sanitation departments.
Prayer services, concerts and private memorial services were planned throughout the day, such as one held by bond trading company Cantor Fitzgerald, whose offices were on the 101st to 105th floors of the north tower.
Cantor and its subsidiaries lost 658 people — accounting for nearly a quarter of those killed in New York that day.
"It was so beyond sadness," Chief Executive Officer Howard Lutnick said in a recent interview. "It will forever hurt."
In Brooklyn, a group of men in blue coveralls applauded as the procession passed. Many in the crowd held red, white and blue candles, which flicked in the pre-dawn fog.
Some held posters of the trade wreckage and the words, "We Will Never Forget."
Officer Jim Coughlan, a bagpiper with the New York Police Department's Emerald Society Pipes and Drums, called the morning a mix of "pride, sadness, mourning and happiness that we're moving on, looking forward to the future, to rebuild."
Similar marches from the Bronx, Manhattan and Staten Island were to end around 7:30 a.m. at the site of the World Trade Center, where 2,801 people died when hijackers felled the twin 110-story towers last Sept. 11.
Security was intense after U.S. Attorney General John Ashcroft announced a move to the second-highest threat level, reflecting a "high risk of terrorist attacks." The city, however, has been at "orange alert" since last year's attacks.
City streets, tunnels, bridges and airports were under heavy guard by police, National Guard, federal agents, armed teams of special operation officers, snipers, canine patrols and bio-terrorism units. Fighter jets flew patrols overhead, while U.S. Navy and Coast Guard ships monitored the harbors.
President Bush was to arrive in the afternoon. The day was set to conclude with the lighting of an eternal flame and candlelight vigils throughout the city.
Silence was to be the signature of a day honoring victims of the attacks that staggered New York one year ago. The city is to fall quiet four times — once for each plane that crashed into the trade center, and once for each tower's collapse.
New Jersey Gov. James McGreevey will read from the Declaration of Independence and Mayor Michael Bloomberg from Franklin D. Roosevelt's "Four Freedoms" speech.
The ceremony was scheduled to last 102 minutes, ending at 10:28 a.m., one year to the minute after the south tower collapsed.
"Our intent," the mayor said, "is to have a day of observances that are simple and powerful."
Across the city, remembrances were planned for nearly every hour of the day — hospitals honoring fallen paramedics, children's choruses singing mournful tributes, churches praying for the lost.
Around the country, thousands of people planned to gather at similar ceremonies deep in symbolism and history, including observances at the Pentagon and the Pennsylvania field where United Flight 93 crashed.
Barbara Minervino of Middleton, N.J., who lost her husband, Louis, said she would not attend the city's ceremony. She said keeping speeches out of the anniversary remembrances was a good idea.
"There are no words, really, that anyone can say, that would heal the heart, that would change the moment, so silence is probably best," she said.
Extraordinary security was planned at bridges, tunnels, landmarks and anniversary ceremonies, although officials cautioned there was no specific threat against any target in the city or state.
"This abundance of caution is something that should be comforting to everyone," Bloomberg said. "My basic advice is to follow what the president of the United States has suggested we do, and that is to leave security to the professionals."
Ground zero was to be the last of the three disaster sites visited by President Bush on the anniversary, and he planned to address the nation from Ellis Island on Wednesday night.
With the Statue of Liberty as his backdrop, Mr. Bush aimed to remind America of "our moral calling, our higher purpose as the beacon of liberty and freedom for people around the world," said White House press secretary Ari Fleischer.