Ground zero in lower Manhattan went quiet at 8:46 a.m. and 9:03 a.m., for the impacts of American Airlines Flight 11 and United Flight 175, and again at 9:59 a.m. and 30 minutes later, when the south and north towers fell.
Mayor Michael Bloomberg, the mayor of New York, paid tribute to those who lost their lives and the rescuers who fought to save them. "Five years have come, and five years have gone, and still we stand together as one," he 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 spouses and partners of the 2,749 people who died at the trade centre then read out the names of the victims - as has happened every year since the attacks.
President George W Bush opened the day by hosting a breakfast for the 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-mast above him. Mr Bush is to visit the attack sites in Shanksville, Pennsylvania, and the Pentagon later in the day before addressing the American public on television from the Oval Office.
In Shanksville, where United Flight 93 crashed to the ground, killing 40, people gathered at a temporary memorial - a chainlink fence covered with American flags, firefighter helmets and children's drawings.
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 are planned around the country and the world.