The 11 September 2001 attacks killed almost 3,000 people in New York, the Washington DC area and Pennsylvania.
In New York, families of the victims are reading the names of each person who died at the World Trade Center.
President Barack Obama was set to attend a memorial outside the Pentagon, but no speeches have been planned by politicians.
The attacks led to a long war in Afghanistan and created an expansion of government surveillance powers that have recently come under intense debate.
A separate memorial will be held outside Shanksville, Pennsylvania, honouring the passengers and crew of United Flight 93, who struggled with the hijackers of the plane, preventing it from hitting its intended target, believed to be the White House or the US Capitol building.
All 33 passengers and seven crew members on the flight were killed after the plane crashed into a field about 75 miles (120 km) south-east of Pittsburgh.
"No matter how many years pass, this time comes around each year, and it's always the same," Karen Hinson, who lost her brother, Michael Wittenstein, in New York, told the Associated Press news agency. His body was never found.
The memorials come as builders put the finishing touches to the new World Trade Center tower and a museum describing and remembering the attacks.
On Tuesday, a groundbreaking was held for a visitor centre for the Flight 93 national memorial park.
The building, expected to open in late 2015, will be broken in two where the plane flew overhead. Visitors have already left 35,000 tributes at the site.
*May the souls of those who died in the attacks continue to rest in peace*
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