Family members gathering at "Ground Zero" clutched roses and photos of the relatives they lost on September 11.
They fell silent at 8:46 a.m., the exact time American Airlines Flight 11 slammed into the World Trade Center's north tower.
There were three other similar moments of silence -- marking the second plane crash, and the time each tower collapsed.
As on the previous anniversaries, the names of the more than 2,700 victims of the trade center attack were read out.
Last year the victims' children did the reading -- this time it was their parents and grandparents.
During the ceremony, families were able to descend to the pit of the towers, seven stories below street level.
It's their last chance to do that before construction starts in earnest on a new skyscraper, the 540-meter-tall Freedom Tower that will form the centerpiece of the redeveloped site.
New York City Mayor Michael Bloomberg spoke at the ceremony of the parents' suffering.
"There is no name for a parent who loses a child, for there are no words to describe this pain," he said.
Memorial services are also taking place in churches and towns across the United States and in several other countries.
In Washington, U.S. President George W. Bush observed a moment of silence on the lawn of the White House.
Later, he gave a live radio address, in which he vowed to continue the hunt for terrorists behind September 11 and other attacks.
"The United States is determined to stay on the offensive, and to
pursue the terrorists wherever they train, or sleep, or attempt to set down roots," Bush said.
One of the last events today will take place at sundown in New York.
Light beams that evoke the twin towers will be projected upward from a lot near the site, to remain on through the night.