RELATED SLIDESHOW: Marking 8 years since the 9/11 attacks
RELATED SLIDESHOW: Images of the 9/11 attacks
Skies were gray in New York City, at the Pentagon and at the crash site of United Airlines Flight 93 in a Shanksville, Pa., field, where now-familiar ceremonies honored the nearly 3,000 people who were lost. Friday was also the first time the anniversary was observed as a national day of service, following an order signed this year by President Barack Obama.
"From this day forward, we will safeguard the memories of those who died by rekindling the spirit of service that lit our city with hope and helped keep us strong," New York Mayor Michael Bloomberg said at a ceremony in lower Manhattan.
Obama and first lady Michelle Obama observed a moment of silence in honor of 9/11 victims outside the White House as a single bugler played taps. A Washington rain came to a stop as the observance began at 8:46 a.m., the moment the first jetliner struck the World Trade Center on Sept. 11, 2001. Later, in the rain, he placed a wreath at the Pentagon.
The nation, he said, came together as one after the terrorist attacks, "united not only in our grief but in our resolve to stand up for the country we love."
At a plaza adjacent to ground zero in New York City, families gathered in the rain, with their umbrellas whipping inside out, while the names of the Trade Center victims were read, pausing for moments of silence at the minutes the jetliners crashed into the towers.
People involved in volunteer work across the nation joined relatives of victims to read the names of those lost in the twin towers.
One reader represented a group called New York Says Thank You, which sends volunteers from New York City each year on the attacks anniversary to help rebuild communities around the country affected by disasters as a way to send thanks for the help that came to New York City after Sept. 11.
Other readers were from local soup kitchens, advocacy groups and well-known service organizations including the American Red Cross and the United Way.
As has become tradition, relatives who read names called out greetings and messages of love to the lost.
"We miss you; life will never be the same without you. This is not the rain," said Vladimir Boyarsky, whose son, Gennady Boyarsky, was killed. "This is the tears."
In New York City, Vice President Joe Biden spoke during a pause in the reading of the names, telling the several hundred victims' relatives gathered that "there's a special fraternity for those of us who've lost spouses and children." Biden's daughter and first wife died in a 1972 automobile accident.
Before he spoke, Biden joined families who were laying flowers in a reflecting pool on the site where the towers once stood.
Relatives and friends of victims were allowed Friday to visit the plaza for the Sept. 11 memorial that is under construction. It is expected to be partially complete and open for the 10th anniversary.
Former President George W. Bush had no public appearances planned Friday, and a spokesman said he would be working in his office during the morning. In a brief statement, he said he and his wife, Laura, were thinking of the victims and their families.
"We honor those who volunteer to keep us safe and extend the reach of freedom - including members of the armed forces, law enforcement officers, and intelligence and homeland security professionals," the statement said. "Their courage, service, and sacrifice is a fitting tribute to all those who gave their lives on Sept. 11, 2001. On this day, let us renew our determination to prevent evil from returning to our shores."
Associated Press writers Verena Dobnik in New York, Nancy Benac in Washington and Dan Nephin in Pittsburgh contributed to this report.